Trim the cauliflower and broccoli into ¾-inch florets. If using broccoli stalks, use only ½-inch of stalk next to the floret.
Fill a Dutch oven or stockpot 2/3rds full with water. Add 1-tablespoon salt and bring the water to a boil. Stir the boiling water and add the cauliflower and broccoli. Cook until the vegetables have a slight bite, 3-4 minutes and drain.
Heat a large skillet over medium heat. Add butter and cook just until foaming subsides.
Add onion and cook until softened and translucent, 2-3 minutes.
Add garlic and cook 30 seconds.
Sprinkle the flour over the onion/garlic mixture; stir and cook 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, salt, red pepper flakes, dry mustard, and black pepper. Cook (and stir) just until mixture has thickened a little.
Add both cheeses and stir until cheese is melted.
Remove from the heat and add the cauliflower and broccoli; stir gently to coat the vegetables
Transfer cauliflower/broccoli mixture to the prepared dish and sprinkle with the breadcrumb topping.
Bake at 425°F for 15-20 minutes or until the sauce bubbles around the edges and the top is golden brown. Serve immediately.