Preheat your oven to 375F and move the rack to the top third of the oven.
Cook the noodles for a minute or two less than the package directions indicate, then drain them and transfer them to a 9x13 casserole dish.
Meanwhile, add the olive oil, onion, and ground beef to a skillet. Cook over medium-high heat, breaking the meat up as you go along, until it's cooked through (about 8-10 minutes).
Stir in the garlic and crushed red pepper flakes and cook for 30 seconds.
If there's a lot of excess fat in the skillet, spoon most of it out, then add in the marinara sauce and let it warm through.
Pour the beef mixture into the casserole dish and add half the cheese and toss together (I also like to add some salt & pepper at this point), then smooth it out in an even layer.
Top the casserole with the remaining cheese and bake, uncovered, for 10-15 minutes or until the cheese is melted and it's nice and hot. Optional: broil for a few minutes (watch it carefully) to brown the cheese.
