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

Meanwhile, preheat oven to 375°F. Lightly spray a casserole dish with cooking spray.

In a medium saucepan over medium heat, melt butter. Add chopped red onion and cook until softened, about 3 minutes. Add minced garlic and cook for 1 minute.

Sprinkle flour over onion mixture and stir for 2-3 minutes.

Gradually whisk in milk and cook until boiling and slightly thickened.

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

Add oregano, marjoram, black pepper, cherry tomatoes, olives, and spinach. Mix well. Taste and adjust salt if needed.

Drain pasta and combine with sauce. Transfer to casserole dish. Top with remaining feta cheese.

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