Prep the chicken and vegetables: Pat the chicken thighs dry with paper towels (dry surface = better sear). Season both sides with 1 tsp kosher salt, 1/4 tsp black pepper, 1/2 tsp smoked paprika, and 1/4 tsp cumin. Let sit at room temperature while you slice the peppers into 1/2" strips and thinly slice the onion. Bringing the chicken closer to room temperature shortens cook time and helps it cook evenly.

Heat your pan: Warm a large heavy skillet (cast iron or stainless steel) over medium-high heat for 2–3 minutes until hot — a properly heated pan is essential to achieve a brown, Maillard crust that adds depth of flavor.

Sear the chicken: Add 1 tbsp olive oil to the pan and swirl. Add the thighs skinless-side down (or smooth-side) in a single layer without crowding — do this in two batches if necessary. Sear undisturbed 4–5 minutes until a deep golden-brown crust forms; flip and sear the other side 3–4 minutes. Searing at high heat quickly develops flavor; avoid moving the meat so the crust can form. Remove chicken to a plate and tent loosely with foil.

Caramelize the onions: Reduce heat to medium, add 1 tbsp olive oil and the sliced onions to the same pan. Sprinkle a pinch of salt (about 1/4 tsp) over them. Cook, stirring occasionally, 8–10 minutes until soft and richly golden. Caramelization transforms the onion's sugars and builds the sweet, savory base for the sauce.

Cook the peppers and aromatics: Add the pepper strips and a pinch more salt to the onions. Stir and cook 6–8 minutes until the peppers begin to soften and develop charred edges; push the veggies to the side and add the butter. Once melted, add the minced garlic, remaining smoked paprika, cumin, oregano, and black pepper; cook 30–45 seconds until fragrant. Toasting the spices in fat blooms their oils and deepens flavor.

Deglaze and build the sauce: Pour in 1/2 cup chicken stock and use a wooden spoon to scrape up the flavorful brown bits (fond) from the bottom of the pan — this is where a lot of the dish's flavor lives. Add the lemon zest and 1 tbsp lemon juice. Simmer 2–3 minutes to reduce slightly and concentrate flavors.

Finish the chicken in the pan: Nestle the seared chicken thighs back into the skillet, spooning some sauce over them. Reduce heat to medium-low and simmer uncovered 6–8 minutes, or until the internal temperature reaches 165°F (74°C). Finishing in the pan allows the chicken to absorb the pepper-onion flavors while the sauce reduces and thickens slightly.

Finish with a glossy glaze and rest: Remove the skillet from heat and swirl in the remaining 1 tbsp butter to enrich and slightly thicken the sauce (mounting with butter gives a silky mouthfeel). Taste and adjust seasoning with salt and pepper. Let the chicken rest 5 minutes — resting redistributes juices so the meat stays moist when sliced.

Serve: Garnish with chopped parsley and an extra pinch of smoked paprika or crushed red pepper if using. Serve family-style straight from the skillet over steamed rice, crusty bread, polenta, or tucked into warm tortillas.