Preheat the oven to 400°F (200°C). Butter an 8x8-inch baking dish or a 9-inch deep pie dish and set aside.
In a small bowl, combine the drained diced tomatoes, chopped olives, 1 tablespoon chopped basil, and dried oregano; set aside.
Bring a large pot of salted water to a boil and cook the elbow macaroni until al dente according to package directions; drain well and return to the pot.
While the pasta cooks, melt 2 tablespoons butter with 2 tablespoons olive oil in a medium saucepan over medium heat.
Add the chopped red onion and sauté until soft and lightly golden, about 3–4 minutes, then add the minced garlic and cook 1 minute more.
Whisk in 3 tablespoons flour and cook, stirring constantly, for 1 minute to form a roux.
Gradually whisk in 2 cups whole milk, whisking vigorously to avoid lumps, and bring the mixture just to a boil.
Reduce heat to medium-low and simmer, stirring constantly, until the sauce thickens, about 3 minutes. Remove from heat and stir in 2 ounces of 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 stir to coat evenly.
Transfer the coated pasta to the prepared baking dish and sprinkle the top evenly with the remaining 4 ounces of crumbled feta.
Bake in the preheated oven for 20–25 minutes, until the edges are bubbling and the top is golden.
Serve warm garnished with additional fresh basil ribbons if desired.
