Preheat the oven to 375° F (190° C). Lightly coat an 8×8, 9×9 or gratin baking dish with oil or butter.
Add 1-2 inches of water into a large pot fitted with a steamer basket. Bring the water to a boil. Carefully add the broccoli and cauliflower, cover, and simmer until just crisp tender, about 5 minutes. Remove the basket from the pot and allow it to drain.
Heat a medium saucepan or skillet to medium heat. Add the olive oil, allow it to warm a bit, and then add the onion and saute until translucent, about 5 minutes. Add the garlic and stir until fragrant, 30 seconds.
Add the flour (or alternative) and stir constantly, until the mixture turns golden and thick.
Slowly add the half and half, about ½ cup at a time, stirring constantly, adding more as it thickens.
Once the sauce has thickened, lower the heat, and add the cheeses in small batches and stir until melted.
Add the nutmeg and stir to combine. Add salt and pepper to taste. I used ½ teaspoon of each.
Add the steamed broccoli and cauliflower to the sauce and stir to coat. Then pour the mixture into the prepared baking dish.
Combine the melted butter, breadcrumbs and 2 tablespoons of parmesan cheese in a small bowl. Sprinkle over the top of the casserole.
Bake for 25 minutes until bubbly and top is slightly golden, serve immediately.