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.
Add the cauliflower and 2 teaspoons salt to a large pot of boiling water and cook for 5 minutes. Drain and rinse with cold water to stop the cooking process.
Add the cooked cauliflower to a 9x13 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-size bowl and whisk well. Add the flour slowly, whisking constantly to prevent lumps. When the flour is incorporated add in the milk and cream. Whisk to combine. Add the salt, pepper, and nutmeg and whisk to combine. Add 1-1/2 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 gratin is puffed and golden and a knife inserted in the center comes out clean. Serve immediately.