Position a rack in the middle of the oven and preheat to 325°F and butter a 9” x 9” baking dish.
Heat the olive oil over low heat in a medium-sized pan. Add in the onions, stir, and tightly cover. Cook for 10 minutes, stirring once or twice in between, and then set aside.
In a small saucepan, melt the butter over medium heat. Add in the flour and whisk until starting to bubble, then cook while continuing to whisk for an additional 10 seconds. Take off the heat and set aside.
Place the milk, nutmeg, rosemary, bay leaf, salt, and pepper in a medium-sized pot set over low heat. Cover and bring to a boil. This may take some time, since the heat is low, however it gives the flavors more time to infuse together.
Once at a boil, add in the cauliflower and cover again. Cook for 5-7 minutes, until the cauliflower is cooked through and a knife can easily be inserted into the stem of a floret. Take out the bay leaf and discard, then add in the roux and continue cooking until the milk is thickened, about 1 minute. Turn off the heat and mix in the Gruyere, 2 tablespoons of parmesan, and sautéed onions.
Pour the cauliflower mixture into the baking dish and even out the top. At this point you can store it in the refrigerator for up to 2 days ahead of time. Let it cool completely, then cover with plastic wrap before placing it in the refrigerator. When you want to reheat it, let it come to room temperature before placing it in the oven. Since it won’t be hot, you may need to leave it for an extra few minutes in the oven.
Sprinkle the 2 tablespoons of leftover parmesan over the top, as well as the Cauli Crunch Cauliflower Crumbs. Cook for 25-30 minutes, until the sides start to bubble. Serve immediately.