Marinate the chicken (optional but recommended). In a bowl or zip-top bag, combine the buttermilk, 1/2 teaspoon kosher salt, 1/4 teaspoon black pepper, and 1/2 teaspoon smoked paprika. Add the chicken thighs, toss to coat, and refrigerate for 1–4 hours (or overnight). WHY: Buttermilk's mild acidity tenderizes the chicken proteins and adds a subtle tang that cuts through the cheese; marinating improves texture and flavour.

Sauté the aromatics. Set a 10–12 inch skillet over medium heat, add the olive oil, then the chopped onion and diced bell pepper. Cook 6–8 minutes until the onion is soft and beginning to brown — this browning (Maillard reaction) creates sweet, savory flavor. Add the garlic and cook 30–45 seconds until fragrant. WHY: Softening and browning aromatics first concentrates their flavor so the slow cooker base will be more complex; garlic becomes bitter if overcooked, so add it last briefly.

Make a light roux to thicken the final sauce. Push the vegetables to one side of the pan, add the butter and let it melt, then sprinkle in the flour and stir 1–2 minutes until it smells toasty and loses its raw flour taste (but don’t let it darken too much). WHY: The flour will add body to the cooking liquid so your rice-cheese mixture finishes creamy instead of soupy.

Deglaze and season. Pour 1 cup of the chicken broth into the skillet, scraping up browned bits from the bottom; then stir in the remaining 2 cups of broth, the smoked paprika, dried thyme, remaining salt and pepper, and optional cayenne. Bring to a gentle simmer for 1 minute. WHY: Deglazing lifts flavorful fond from the pan into the liquid; pre-seasoned broth distributes flavor through the rice during slow cooking.

Assemble the slow cooker. Spread the uncooked rice in an even layer on the bottom of a 6-quart slow cooker (this helps rice cook evenly). Scatter the sautéed onion/pepper mixture over the rice. Pour the hot broth mixture over the rice and vegetables. WHY: Placing rice on the bottom ensures even absorption of liquid and consistent cooking; starting with hot liquid shortens the time the cooker needs to come to temperature.

Nestle the chicken into the rice. Remove the chicken from the buttermilk (drip off excess — do not wipe), season lightly with a little extra salt and pepper, and place the thighs on top of the rice, spaced apart. Cover and cook on LOW for 2 to 2 1/2 hours (check at 2 hours). WHY: Cooking the chicken on top steams it gently while the rice absorbs the broth — this method prevents the rice from sticking to the chicken and allows juices to flavor the rice without overcooking the meat.

Check for doneness. At ~2 hours, test a rice grain — it should be tender but not mushy. Use an instant-read thermometer in the thickest part of a thigh; chicken is safe at 165°F (74°C). If the rice is still firmer than you'd like, continue cooking in 15–20 minute increments. WHY: Slow cooker models vary; early checking avoids overcooked, mushy rice while ensuring safety and tenderness.

Shred the chicken. Remove the thighs to a cutting board, shred with two forks, then return the shredded meat to the slow cooker and stir gently to distribute. WHY: Shredding makes the dish cohesive so every spoonful has chicken; returning the juices helps keep the rice moist and flavorful.

Temper and finish with cheese. Reduce the cooker to HIGH. In a small bowl, whisk the remaining 1/2 teaspoon salt into the buttermilk (if any remained) and then add it slowly to the cooker while stirring — if you used all the buttermilk for marinating, simply add 1/3 cup warm broth instead. Stir in 1 1/2 cups of the shredded cheddar and all the mozzarella and Parmesan. Cook uncovered on HIGH 8–12 minutes until the cheese melts into a glossy, creamy sauce. WHY: Adding dairy at the end prevents curdling and lets the cheese melt smoothly into the rice; cooking uncovered concentrates the sauce slightly.

Finish and rest. Taste and adjust seasoning (salt, pepper, a squeeze of lemon or a tablespoon of hot sauce if you want brightness). If the mixture is thinner than you prefer, mix 1 tablespoon cornstarch with 1 tablespoon cold water to make a slurry and stir it in, then cook on HIGH 5 more minutes. Turn off the cooker and let rest 10 minutes with the lid off — the sauce will set slightly. WHY: Resting allows the starches to relax and the sauce to thicken evenly; finishing adjustments brighten and balance the rich, cheesy flavors.

Serve. Spoon generous portions into bowls or onto plates, sprinkle the reserved 1/2 cup shredded cheddar over each serving for a melty finish, and add chopped fresh parsley for color. WHY: Fresh herbs and an extra sprinkle of cheese add visual appeal and a flavor layer that completes the dish.

Optional crisp top (restaurant touch). For a gratin-style finish, transfer portions to an oven-safe dish, broil 2–3 minutes to brown the top (watch closely). WHY: Broiling adds texture contrast — a crisp cheesy top complements the creamy interior, lifting the dish to a restaurant-style presentation.