Step 1

Preheat oven to 350°F. Coat 9-by-13-inch baking dish with cooking spray.

Step 2

Bring 1 inch of water to a boil in a large pot fitted with a steamer basket. Place broccoli in the basket and steam until just tender, 4 to 6 minutes. (Alternatively, place broccoli in a large microwave-safe bowl with 2 tablespoons water. Cover and microwave on High until the broccoli is tender-crisp, 5 to 8 minutes, stirring once.) Coarsely chop the broccoli and spread evenly in the prepared baking dish. Remove the steamer basket and pour out any remaining water from the pot.

Step 3

Add oil and 1 tablespoon butter to the pot; heat over medium-high heat. Add onions and cook, stirring often, until soft and translucent, 3 to 5 minutes. Add mushrooms, garlic and crushed red pepper; cook, stirring occasionally, until the mushrooms are soft and all the liquid is released, about 5 minutes. Sprinkle flour evenly over the vegetables; cook, stirring constantly, for 1 minute. Slowly add broth, stirring constantly. Cook, stirring occasionally, until thickened, 3 to 5 minutes. Stir in cream cheese, lemon juice, pepper and 1/2 teaspoon salt; cook, stirring constantly, until smooth, about 2 minutes. Remove from heat and stir in 3/4 cup Parmesan. Pour the mushroom sauce evenly over the broccoli, stirring gently to combine.

Step 4

Microwave the remaining 2 tablespoons butter in a small microwave-safe bowl on High until melted, about 30 seconds. Add panko and the remaining 1/4 cup Parmesan and pinch of salt; stir to combine. Sprinkle evenly over the broccoli mixture.

Step 5

Bake the casserole until the cheese is melted and the sauce is bubbling around the edges, 25 to 30 minutes.