Preheat the oven to 375 degrees.
Begin by making a roux. Melt 2 tablespoons of butter in a medium saucepan over low heat. Add the flour, stirring constantly with a wooden spoon for 2 minutes, until lightly browned and fragrant.
Slowly pour the warmed heavy cream into the roux and whisk constantly but slowly until hot but not boiling. Continue whisking constantly, for 1 minute, or until thickened. Remove from heat and add 1 teaspoon of salt, pepper, nutmeg, 1/2 cup of the cheddar, and the Parmesan.
Pour 1/3 of the sauce on the bottom of an 9 x 11 baking dish. Place the drained cauliflower on top and then pour the rest of the sauce evenly over the cauliflower. Combine the bread crumbs with the remaining 1/4 cup of cheddar and sprinkle on top. Sprinkle with salt and pepper. Bake for 25 to 30 minutes, until the top is browned. Serve hot or at room temperature.