Preheat the oven to 190°C / 375°F. Position a rack in the middle to ensure even browning on top. While the oven warms, you’ll sear the chicken and assemble the casserole so the oven finish crisps the topping.

Rinse the rice briefly under cold water until the water runs clear, then drain. Rinsing removes excess surface starch so the cooked rice stays distinct and won’t glue the casserole into a gummy mass.

Season the chicken breasts on both sides with 1 tsp Kosher salt, 1/2 tsp black pepper, and half the dried thyme. Heat 1 tbsp olive oil in a large skillet over medium-high heat until shimmering—this temperature helps create a golden crust quickly without overcooking the interior.

Sear the chicken 2–3 minutes per side until nicely browned (it will not be fully cooked). Browning creates Maillard flavor that deepens the overall dish. Transfer the chicken to a plate and tent with foil to rest while you build the sauce.

In the same skillet, add the remaining 1 tbsp olive oil and 1 tbsp butter. Reduce heat to medium, add the diced onion and a pinch of salt, and cook until translucent (about 4–5 minutes). Stir in the minced garlic for 30 seconds until fragrant. Cooking the aromatics in the fond from searing captures flavor from the chicken.

Make a quick roux: add the remaining 1 tbsp butter and the 2 tbsp flour to the skillet. Cook, stirring, for 60–90 seconds to eliminate raw flour flavor and to toast slightly — this improves the sauce’s depth. Slowly whisk in the chicken broth and then the milk to avoid lumps; bring to a gentle simmer. Simmer 3–4 minutes to thicken to a loose cream sauce. The roux thickens the sauce and keeps the rice creamy without getting soggy.

Stir in 1 cup (100 g) of the shredded cheddar, remaining thyme, 1/2 tsp Kosher salt, and 1/4 tsp black pepper. Taste and adjust seasoning; the cheese should melt smoothly into the sauce. Remove from heat—this prevents the cheese from breaking and becoming grainy.

Combine the rinsed uncooked rice with the sauce in a large mixing bowl (or directly in a 9 x 13-inch casserole dish). Stir to coat the rice so each grain begins absorbing the seasoned milk mixture. This step ensures even cooking and flavor distribution.

Slice the seared chicken into 1/2-inch thick strips and nestle them evenly into the rice-sauce mixture, pressing them slightly into the rice so they finish cooking inside the casserole. Scatter any accumulated juices from resting over the rice — those juices add concentrated flavor.

Sprinkle the remaining 1/2 cup (50 g) shredded cheddar evenly over the top. In a small bowl, toss the panko breadcrumbs with the lemon zest (if using), a pinch of salt, and 1 tbsp melted butter, then scatter over the cheese for a golden, crunchy topping. The contrast of creamy interior and crisp top elevates the mouthfeel.

Bake uncovered at 190°C / 375°F for 35–40 minutes, until the rice is tender and the internal temperature of the chicken reaches 74°C / 165°F. If the top is browning too quickly, tent loosely with foil. Baking finishes the rice and melds flavors while producing a browned topping.

Remove from oven and rest 8–10 minutes before serving. Sprinkle chopped parsley and an extra grind of black pepper. Resting lets the sauce settle so servings hold together and prevents burnt mouths from very hot pockets of sauce.