Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a 2-quart baking dish.

Heat oil in a large skillet over medium-high heat. Add onion, bell pepper, mushrooms, 1/4 teaspoon salt, and black pepper; cook and stir until begins to brown, 12 to 15 minutes.

Sprinkle flour over mushroom mixture; stir until well combined, about 1 minute. Add milk and remaining 1/4 teaspoon salt; bring to a boil, stirring frequently, until slightly thickened, about 3 minutes. Stir in broccoli, Cheddar cheese, and 2 tablespoons Romano cheese. Transfer to the prepared baking dish.

Toss bread crumbs with remaining 2 tablespoons Romano cheese; sprinkle over casserole and dot with butter.

Bake in the preheated oven, uncovered, until top is golden and casserole is bubbling, about 40 minutes. Cool before serving, 5 minutes.