Preheat your oven to 350°F (175°C). Ensure both the cooked macaroni and steamed cauliflower are drained very well to prevent a watery sauce. If cooking the macaroni ahead of time, toss it with a little oil to prevent sticking.
In a medium saucepan, melt the butter over medium heat.
Whisk in the flour to form a paste (a roux) and cook for 2 minutes, whisking constantly. This step cooks out the raw flour taste.
Gradually whisk in the milk until smooth.
Continue cooking, whisking frequently, for about 7 minutes or until the sauce thickens and begins to bubble.
Remove the saucepan from the heat.
Stir in the grated Cheddar and Monterey Jack cheeses until fully melted and the sauce is smooth.
Season with salt and pepper.
In a 10x10-inch or similar-sized baking dish, combine the cooked macaroni and steamed cauliflower.
Pour the cheese sauce over the top and toss gently to coat everything evenly.
Sprinkle the bread crumbs over the surface of the casserole.
Bake for 20 to 25 minutes, until the sauce is bubbly and the top is golden brown.
Let it rest for a few minutes before serving.