1 large head cauliflower, trimmed, halved, and cut into 1/4-inch-thick slices
8 ounces cremini mushrooms, halved
3 cloves garlic
8 to 10 sprigs thyme, plus additional for garnish
Extra-virgin olive oil, to coat
Kosher salt, divided
Cracked black pepper
2 1/4 cups milk
1/2 cup heavy cream
2 large shallots, diced
1 tablespoon sage, plus additional for garnish
1/4 teaspoon nutmeg
1 1/2 cup aged Parmigiano-Reggiano, divided
1 1/2 cups tater tots, pan-fried and broken up
1/4 cup pine nuts, toasted (optional)
Preheat oven to 375°F. On a large baking sheet, arrange the cauliflower, mushrooms, and garlic. Make sure not to crowd the pan; use two pans if needed. Toss the thyme sprigs evenly around the pan. Pour on enough olive oil to coat (a few tablespoons). Season generously on both sides of the vegetables with salt and pepper. Bake in the middle of the oven for 15 to 20 minutes and then broil for 3 to 5 minutes. The cauliflower should just be starting to char. Remove from the oven. Remove the skin from the garlic and mince. Coarsely chop the cauliflower.
In a medium saucepan combine the milk, heavy cream, shallots, minced roasted garlic, sage, and nutmeg. Bring to a boil over medium high meat. Turn down to low and allow to simmer for 10 to 12 minutes. The sauce should reduce and thicken a bit. Remove from the heat and stir in 1/2 cup of Parmesan and salt to taste. Set aside.
In a round 10-inch dish, arrange half of the cauliflower and mushrooms into a tight single layer. Cover with half of the sauce, top with 1/2 cup of Parmesan cheese. Repeat with the rest of the ingredients.
Bake on lower rack at 375°F, covered for 20 minutes. Top with the browned tater tots and broil for 5 minutes. Sprinkle over with the toasted pine nuts, if using, and a fresh sprinkling of thyme and sage.