Cook noodles according to package instructions. While the noodles are cooking, make the sauce.*
3 cups medium-width egg noodles (uncooked)
Heat the oil in a pan until shimmering.
1 tablespoon mild olive oil
Add onion and cook until starting to soften (about 2 minutes).
¼ cup minced yellow or white onion
Add mushrooms and cook, stirring occasionally, until most of their liquid is released (about 5 minutes).
6 ounces cremini or white button mushrooms, stems removed, caps quartered
Stir in garlic, thyme and pepper, then stir in the flour.
2 garlic cloves, peeled and minced or pressed through a garlic press, ½ teaspoon dried thyme, ¼ teaspoon ground black pepper, 2 tablespoons all-purpose flour
Pour in broth and cream, then bring to a full boil. Cook, stirring continuously, until the sauce has thickened enough to coat the back of a spoon (12 to 15 minutes). Turn the heat off, add the raw spinach and stir a few times just until starting to wilt. Set aside.
2 cups chicken broth, 1 cup heavy cream, 2 cups spinach leaves
Heat frozen meatballs in the microwave according to package instructions.
Stir cooked noodles, meatballs and sauce together. Transfer to a baking dish and top evenly with cheese.
2 cups grated sharp white cheddar cheese
Bake until the cheese is fully melted (10 to 15 minutes). Sprinkle with fresh thyme leaves (optional). Serve hot with garlic bread and green salad.
Recipe notes:
*You can make the sauce up to a day ahead and keep in an airtight container in the fridge. Briefly heat it up in the microwave or on the stovetop when you're ready to make the casserole.Storage: Store leftovers either in the casserole dish covered with plastic wrap or in any other airtight container in the fridge. Reheat in the microwave or in the oven.To make 8 portions: This recipe makes 4 portions in an 8-inch by 8-inch baking dish. If you want to double the recipe, use a 9-inch by 13-inch baking dish instead and double all ingredients.