Preheat the oven to 400°F (200°C) and butter an 8x8-inch baking dish or a 9-inch deep pie dish; set aside.
In a mixing bowl, combine the drained diced tomatoes, chopped olives, 1 tablespoon chopped basil, and 1/2 teaspoon dried oregano; set aside.
Bring a large pot of salted water to a boil and cook 8 oz elbow macaroni until al dente according to package directions; drain well and return to the pot.
Meanwhile, melt 2 tablespoons butter with 2 tablespoons olive oil in a medium saucepan over medium heat. Add 1/3 cup chopped red onion and sauté 3–4 minutes until soft and lightly golden, adding the minced garlic during the last minute.
Whisk in 3 tablespoons flour and cook, stirring constantly, for 1 minute. While whisking, slowly pour in 2 cups milk and bring just to a boil, then reduce heat and simmer, stirring, until thickened, about 3 minutes.
Remove the sauce from heat and stir in 2 ounces of the crumbled feta and the shredded mozzarella. Season with salt and freshly ground black pepper to taste.
Toss the drained pasta with the tomato-olive mixture in the pot, then pour the cheese sauce over the pasta and toss until evenly coated.
Transfer the coated pasta to the prepared baking dish, then sprinkle the remaining 4 ounces crumbled feta evenly over the top.
Bake in the preheated oven for 20–25 minutes, until the edges are bubbling and the top is golden brown.
Serve warm, garnished with additional fresh basil ribbons if desired.