Cut the head of cauliflower in half and then into quarters through the core. Then, run your knife between the florets and the stem and cut the stem out. Discard the stem and cut any large florets into smaller pieces.
To par-cook the cauliflower, add it to a large microwave-safe bowl, cover it, and microwave on high for four to five minutes. Drain any liquid. (Another option) You can also toss it in olive oil and roast it in a 425 °F oven for about 15 minutes. Either way, you want the cauliflower to be slightly undercooked. It will continue to cook when it's baked.
Add the cooked cauliflower to an 8X8 baking dish, which has been sprayed with non-stick cooking spray. Sprinkle the bacon over the cauliflower. Set aside.
Add the eggs to a medium-sized bowl and whisk well. Add the flour slowly, constantly whisking to prevent lumps. When the flour is incorporated, add the milk and cream. Whisk to combine. Add the salt, pepper, and nutmeg and whisk to combine. Add one and a half cups of the gruyere cheese and mix well.
Pour the milk and egg mixture evenly over the cauliflower and sprinkle the remaining cheese over the top.
Bake for 25-30 minutes until the casserole is puffed and golden and serve immediately.
Notes
This cauliflower cheese bake is substantial enough to serve as a meatless or vegetarian main dish.
Instead of par-cooking the cauliflower in the microwave or roasting it, another option is to add it and two teaspoons of salt to a large pot of boiling water and cook for five minutes. Then, drain and rinse with cold water to stop the cooking process. Also, drain it again before adding it to the dish.
Leftovers will keep covered in your fridge for up to four days. I don't recommend freezing them. You can reheat leftovers in the microwave for a minute or so.
You can make the casserole as directed up to eight hours beforehand, but do not bake it. Cover and store it in the fridge. When you are ready to bake it, remove it from the refrigerator and allow it to come to room temperature before baking it. You may need to add a few minutes to the total baking time.