Preheat 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. Cook onion, bell pepper, mushrooms, 1/4 teaspoon salt, and black pepper, stirring frequently, until mixture begins to brown, 12 to 15 minutes.
Sprinkle flour over mushroom mixture and cook, stirring, 1 minute. Add remaining 1/4 teaspoon salt and milk and bring to a boil, stirring until smooth. Boil, stirring frequently, until slightly thickened, about 3 minutes. Stir in cheddar and 2 tablespoons romano. Stir in broccoli and pour mixture into prepared baking dish.
Toss bread crumbs with remaining 2 tablespoons romano and sprinkle over top. Dot with butter.
Bake, uncovered, until crumbs are golden and casserole is bubbling, about 40 minutes. Cool 5 minutes before serving.
A trick to popping the flavor of the Cheddar is to add a bit of romano or Parmesan cheese.
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