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1
Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
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2
Melt 1 tablespoon butter in a saucepan over medium heat. Whisk in flour and cook, stirring, until lightly browned, 1 to 2 minutes. Slowly pour in milk, stirring constantly to avoid lumps. Simmer until sauce is thickened, 2 to 3 minutes. Remove from heat. Stir in processed cheese, heavy cream, 1/4 teaspoon salt, and Italian seasoning.
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3
Prick whole cauliflower in several places with a fork. Bring a large pot of lightly salted water to a boil. Add cauliflower and cook, uncovered, until slightly tender, about 3 minutes. Drain.
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4
Melt 2 tablespoons butter in a large skillet over medium-low heat. Add onion; cook and stir until translucent, about 5 minutes. Add potato, carrot, red bell peppers, peas, and green beans. Season with salt. Increase heat to high and cook, stirring frequently, until vegetables start to release their moisture, 5 to 10 minutes. Stir in 1 teaspoon crushed black peppercorns.
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5
Transfer vegetables to prepared baking dish. Place whole cauliflower in the center. Pour some sauce over cauliflower; fold remaining sauce in with vegetables. Sprinkle bread crumbs on top. Scatter Parmesan cheese on top. Sprinkle remaining 2 teaspoons peppercorns over cheese.
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6
Bake in the preheated oven until golden brown on top, about 20 minutes.
