Step 1

Arrange a rack in the middle of the oven and heat to 400°F. Lightly coat a 3-quart casserole dish or a 9x13-inch baking dish with olive oil; set aside.

Step 2

Bring a large pot of salted water to a boil. Add the pasta and cook pasta for 8 minutes, or 2 minutes less than suggested on package instructions, until it's quite al dente, as it will keep cooking when baked. Reserve 1/2 cup of pasta water, then drain and set aside.

Step 3

Heat the olive oil in a large pasta pot over medium heat until shimmering. Add the garlic and red pepper flakes and cook until fragrant, about 1 minute. Increase heat to medium-high, add the tomatoes and their juices, salt, pepper, and 1/2 cup reserved pasta water. Stir to combine, then add the spinach and cook until just wilted, 1 to 2 minutes. Remove from the heat and stir in the meatballs and drained pasta.

Step 4

Pour half of the pasta mixture in the prepared baking dish. Sprinkle with half of the mozzarella and half of the fontina cheese. Top with the remaining pasta mixture and sprinkle with the remaining cheeses. Bake until the top is lightly golden and the cheese is bubbling, 20 to 25 minutes. Let cool for about 10 minutes before serving.