Make-ahead pasta recipes are so easy to prepare for a crowd, and particularly perfect for a delicious milchig Shavuos meal. You can easily refrigerate this before baking and just pop it into the oven or toaster oven until heated through and the top is browned.
Preheat oven to 350°F (180°C). In a large pot of salted, boiling water, prepare pasta according to package instructions. Reserve a cup of pasta water before draining pasta.
In a large pot over low heat, melt butter. Add ¾ cup of milk, garlic powder, and cheeses and stir until all the cheeses have melted and the sauce is smooth and elastic. If you would like a looser sauce, add the remaining ¼ cup of milk. If you would like a bit of a tighter sauce, add ¼ cup of reserved pasta water. Season with salt, if necessary, and freshly ground pepper. If adding nutmeg, do so now as well.
Remove the sauce from heat. Carefully stir in the pasta and spinach, so pasta is coated with the sauce.
Pour the pasta into a 9-inch round dish and sprinkle the panko crumbs on top. Bake uncovered for 15 minutes, or until the cheese is bubbling and the crumbs are browned.