Remove the outer leaves from the cauliflower head. Using a sharp knife, remove the stalk. Then remove the florets by cutting them from the core.
If florets are too large, cut them into smaller pieces.
Soak florets in salt water (or vinegar water) for few minutes. Drain.
Use your favorite method. To steam : add water to the pot, about 2 inches. Bring the water to full boil over high heat. Place cauliflower florets in the steamer basket. Place the steamer over the pot. Cover and cook for about 3 minutes. (See notes below for boiling or microwave).
Don't overcook. They should remain crisp.
Heat oil in a large skillet over medium-low heat. Add onion, carrot and celery. Cook stirring from time to time, until softened, about 15 minutes.
Increase the heat to medium and add ground beef. Cook stirring, and breaking up the meat with a wooden spoon, to brown evenly. About 10 minutes. Spoon off any fat.
Add flour or corn starch. Cook stirring for about 2 minutes.
Add broth, spices, herbs, salt and pepper to taste. Bring to a boil, reduce the heat to low and simmer, stirring occasionally, until thickened, about 5 minutes. Let it cool.
In a saucepan, combine milk, melted butter, flour or cornstarch, nutmeg, salt and pepper. Whisk until mixture is smooth.
Place saucepan on the stove over medium heat. Cook stirring until béchamel is thickened.
Pour meat meat mixture in a baking dish. Place drained cauliflower florets over the top.
Cover with béchamel sauce.
Sprinkle with shredded cheese.
Bake in a preheated oven (400°F | 200°C), until top is browned, about 25 minutes.
Let cool slightly then serve directly from the baking dish.
Other vegetables: I like substituting half the cauliflower for broccoli florets or potatoes cut in small pieces and pre-cooked. Sweet potatoes or yam are also good to add some carbohydrates to the gratin.
And feel free to use cauliflower florets or mashed cauliflower (mixed with mashed potatoes if you have any leftover).
Other meat: use other meat instead of beef, for example ground lamb, turkey or veal. You can also use cooked meat leftovers (roasted chicken).
Béchamel sauce: substitute milk for cream if you like. You can also use Mornay sauce instead of Béchamel. Mornay sauce is Béchamel with cheese.
Meat mixture: add other vegetables in your meat mixture, for example peas, spinach or chard, kale, zucchini or eggplant...
Breadcumbs: before baking, sprinkle with seasoned breadcrumbs for a crispy topping.
Shredded cheese : Gruyère cheese, comté, Emmental or parmesan. In the US, these are less expensive: Cheddar, Monterey Jack an Mozzarella.
Two other methods to cook cauliflower:
For a quick and easy weeknight meal, you can cook cauliflower florets, beef mixture and/or Béchamel sauce a day or two before, even a little longer. Keep everything in the fridge.
Assemble and bake less than 40 minutes before serving. Don't forget to preheat your oven to 400°F | 200°C.