Preheat oven to 425°F and set the oven rack to the middle position. Spray a 2-quart (11×7-inch) casserole dish with nonstick cooking spray.

Tear each slice of bread into 4 pieces.

Place the bread, butter, salt, and pepper in a food processor. Process until coarse crumbs form, about ten 1-second pulses. Set aside.

Trim the cauliflower and broccoli into ¾-inch florets.

Fill a Dutch oven or stockpot with water, add 1 tablespoon of salt, and bring to a boil. Add the cauliflower and broccoli. Cook until the vegetables have a slight bite, 3-4 minutes, then drain.

Heat a large skillet over medium heat. Add butter and cook until foaming subsides.

Add the onion and cook until softened and translucent, 2-3 minutes.

Add the garlic and cook for 30 seconds.

Sprinkle the flour over the onion and garlic mixture; stir and cook for about 1 minute.

Reduce the heat to medium-low and whisk in the cream and broth (or wine). While stirring, bring to a boil.

Reduce heat to low and add nutmeg, dry mustard, red pepper flakes, and black pepper. Cook and stir just until the mixture thickens slightly.

Add both cheeses and stir until melted.

Remove from heat and add cauliflower and broccoli; stir gently to coat the vegetables.

Transfer the mixture to the prepared dish and sprinkle with the breadcrumb topping.

Bake at 425°F for 15-20 minutes or until the sauce bubbles and the top is golden brown. Serve immediately.