Bring a large pot of salted water to a boil. Cook cauliflower until just crisp-tender, about 2 minutes. Drain and run under cold water to stop the cooking process. Drain again and set aside.

Heat oil in a large skillet over medium heat. Add sausage and cook, breaking up with a spoon, until cooked through, about 10 minutes. Add onions, garlic, and thyme; cook, stirring frequently, until onions are soft, about 5 minutes. Add tomatoes, nutmeg, salt and pepper. Bring to a simmer, then stir in spinach and cauliflower. Cook until spinach is just wilted, 2 to 3 minutes.

Spread cauliflower mixture in a lightly greased 2-quart baking dish. Mix together breadcrumbs and cheese and sprinkle evenly over the top of the casserole. Drizzle lightly with olive oil.

To bake right away: Preheat oven to 350° F. Wrap casserole in foil and bake, covered, for about 30 minutes. Remove foil and bake until the cheese-breadcrumb topping is bubbly and beginning to lightly brown, about 10 additional minutes.

If freezing: Allow casserole to cool, wrap dish in foil, label and freeze. To bake after freezing, preheat oven to 350° F. Bake casserole, covered and unthawed, for about 50 minutes. Remove foil and bake until the cheese-breadcrumb topping is bubbly and beginning to lightly brown, about 10 additional minutes.