Preheat oven to 400°F (200°C) and butter an 8x8-inch baking dish or a 9-inch deep-dish pie pan; set aside.
In a small bowl, combine the drained diced tomatoes, chopped olives, 1 Tbsp chopped basil, and 1/2 tsp 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 Tbsp butter with 2 Tbsp 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; add the minced garlic and cook 1 more minute.
Whisk in 3 Tbsp flour and cook, stirring constantly, for 1 minute. Slowly pour in 2 cups milk while whisking vigorously, then bring 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 oz of the crumbled feta and the shredded mozzarella. Season with salt and 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 until evenly coated.
Transfer the coated pasta to the prepared baking dish, sprinkle the remaining 4 oz crumbled feta evenly over the top, and bake for 20–25 minutes until edges are bubbling and the top is golden.
Let cool slightly, garnish with additional basil ribbons if desired, and serve warm.