Step 1Heat the oven to 450 degrees. Place four 2-cup gratin dishes on a foil-lined baking sheet.
Step 2Bring 1 cup water to a simmer in a large skillet over medium heat. Add the cauliflower, season with salt and cover the skillet. Cook until the cauliflower florets are just tender enough to be pierced with the tip of a knife, about 2 minutes. Uncover and continue cooking until the water evaporates and the cauliflower is fully tender, 4 to 5 minutes more. Divide the cauliflower among the prepared gratin dishes.
Step 3In a medium bowl, break apart the ground beef with your fingers into pieces about the size of a dime. Dollop the mustard over the beef then season liberally with salt and pepper. Use your fingers to gently toss the beef until it’s well-coated with the seasonings. Roll walnut-size pieces of beef into about 32 balls (1¼-inch-diameter) and divide them among the gratin dishes, resting them on the cauliflower.
Step 4Pour ½ cup cream over the meatballs and cauliflower in each dish, season with salt and pepper, then top each with ¾ cup grated cheese. Bake until the meatballs are cooked through and the cheese is golden brown and bubbling on top, about 30 minutes. Serve while warm.
Curry Cream: Stir 1 tablespoon curry powder into the cream before dividing it among the gratin dishes.
Lamb and Feta: Swap ground lamb for the beef, omit the Gruyère, and divide 8 ounces crumbled feta among the dishes after baking.
Broccoli-Cheddar: Swap broccoli for the cauliflower and swap sharp cheddar for the Gruyère.