Preheat oven to 450 degrees. Line a baking sheet with foil. Put peppers, cut side down, on the surface. Drizzle with 1 Tablespoon of olive oil and season with salt and pepper. Roast in the oven for 4-6 minutes.
In a large skillet heat the other tablespoon of olive oil. Add onion and sauté for a few minutes. Then add beef and garlic salt and pepper. Cook beef and crumble. Drain well. Let cool a few minutes.
Add mint, oregano, both diced tomatoes, couscous and half of the feta cheese. Gently toss together.
In a baking dish, place peppers cut side up this time and fill with meat and cheese mixture. Top with remaining one cup of feta. Bake at 375 degrees for an additional 10-15 minutes or until heated through.