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1
Spray a skillet with cooking spray and place over medium heat; cook chicken in the hot skillet until no longer pink in the center and the juices run clear, 6 to 8 minutes per side.
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2
Set chicken aside.
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3
Preheat oven to 350 degrees F (175 degrees C).
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4
Spray a 9x13-inch casserole dish with cooking spray.
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5
Place broccoli in the prepared casserole dish.
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6
Melt 3 tablespoons butter in a saucepan over medium heat; stir flour into the melted butter until combined and thickened, 3 to 5 minutes.
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7
Slowly pour in chicken stock and milk, stirring constantly; continue cooking and stirring until sauce is thickened, 5 to 10 more minutes.
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8
Stir onion, red bell pepper, serrano chile pepper, habanero pepper, and carrot into the sauce; simmer until vegetables are slightly softened, 5 to 10 minutes.
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9
Cut the cooled chicken into cubes and sprinkle over broccoli.
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10
Pour sauce over chicken and broccoli.
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11
Heat 1 teaspoon butter in a skillet over medium heat; cook and stir mushrooms until tender, 5 to 10 minutes.
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12
Spoon over sauce layer in the casserole dish.
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13
Sprinkle bread crumbs and Parmesan cheese over mushroom layer.
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14
Bake in the preheated oven until cooked through and bubbling, about 30 minutes.
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15
Turn on the oven's broiler; broil casserole until topping is browned, 2 to 4 minutes.
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16
Spoon casserole into 4 bowls and sprinkle 1/4 cup arugula atop each serving.
