Preheat the oven to 190°C (375°F). Preheating ensures the casserole begins cooking evenly the moment it goes in and helps the crust brown properly.

Pat the chicken breasts dry and season both sides with 1 tsp salt and 1/2 tsp black pepper. Drying the surface promotes better browning when searing, which develops deeper flavor through Maillard reaction.

Heat 1 tbsp olive oil and 1 tbsp butter in a large skillet over medium-high heat until shimmering. Sear the chicken 3–4 minutes per side until golden but not fully cooked through (internal temp ~60°C / 140°F). Transfer to a plate. Searing adds caramelized flavor and forms a protective crust so juices stay in the meat during baking.

Reduce heat to medium, add remaining 1 tbsp olive oil and 1 tbsp butter to the skillet. Add the diced onion and a pinch of salt and sweat 6–7 minutes until translucent and beginning to color. Sweating draws out moisture and concentrates sweetness without burning.

Add the minced garlic and smoked paprika, stirring 30 seconds to 1 minute to bloom the spice and release aromatic oils — don’t let the garlic brown or it will turn bitter.

Sprinkle the flour over the aromatics and stir to form a roux; cook 1–2 minutes. Cooking the flour removes the raw starch taste and gives the sauce body.

Gradually whisk in the chicken broth and then the milk so the sauce becomes smooth. Add Dijon, thyme, rosemary, remaining salt and pepper. Bring to a gentle simmer — the liquid will thicken into a silky cream sauce as the starch from the roux swells.

Slice the potatoes 4–5 mm (⅙–¼ inch) thick. Uniform slices ensure even cooking so some pieces don't under- or overcook.

Lightly butter a 9x13-inch (23x33 cm) casserole dish. Layer half of the potato slices in an overlapping single layer; this creates a stable bed that will cook evenly and soak up flavor from the sauce.

Cut the seared chicken into 2–3 large pieces each (or leave whole if you prefer) and arrange half of the chicken over the potato layer. Pour half of the sauce over the chicken and potatoes so the starches start to absorb the sauce; this insures every bite is saucy and seasoned.

Repeat with remaining potatoes, chicken and sauce to build a second layer. Distribute the shredded cheddar evenly over the top — the cheese melts into the topping to add richness and flavor.

Mix the panko with the remaining 1 tbsp melted butter and a pinch of smoked paprika. Sprinkle the panko over the cheese to form a crispy, golden crust; the fat in the butter encourages deep browning.

Cover the dish tightly with foil and bake at 190°C (375°F) for 35 minutes. Covering traps steam to finish cooking the potatoes and chicken without drying them out.

After 35 minutes remove the foil and bake another 20–25 minutes until the top is golden and the potatoes are tender when pierced with a knife. If the topping browns too quickly, tent loosely with foil. Test the chicken temperature — it should reach 74°C (165°F) in the thickest piece.

Rest the casserole for 10 minutes before serving. Resting lets the sauce thicken slightly and makes slicing neater; finish with chopped fresh parsley for brightness.