Bring a large pot of salted water to a boil. When boiling, add macaroni and cook until just under al dente according to package directions.

Preheat oven to 375°F. Spray a deep casserole dish with cooking spray and set aside.

In a medium saucepan over medium-high heat, melt the butter.

Add chopped red onion and sauté until softened, about 3 minutes. Add minced garlic and sauté 1 minute more.

Sprinkle flour over the mixture and stir, cooking for 2-3 minutes.

Gradually add milk, whisking constantly until boiling.

Remove from heat and stir in mozzarella and one-third of feta until melted.

Add oregano, marjoram, pepper, diced tomatoes, olives, and spinach. Stir to combine and add salt as needed.

Drain pasta and transfer to casserole dish. Pour in cheese mixture, and top with remaining feta.

Bake for 20-22 minutes until top is golden. Garnish with dill or parsley before serving.