Spaghetti Squash Au Gratin

small spaghetti squash (~2 pounds), cut in half lengthwise, seeds and membrane scooped out

olive oil or butter

diced sweet onion

garlic cloves, minced

red pepper flakes (optional)

salt and pepper

plain Greek yogurt (can sub sour cream)

heaping cup freshly grated sharp cheddar cheese

Parmigiano Reggiano