Prepare the peppers:
Preheat your oven to 375°F (190°C). Slice the tops off the bell peppers and remove the seeds. Place the peppers in a baking dish and set aside.

Cook the filling:
In a large skillet, heat olive oil over medium heat. Add the chopped onion and garlic, sautéing for 2–3 minutes until softened. Add ground turkey (or chicken) to the skillet, season with oregano, cumin, salt, and pepper. Cook until browned and fully cooked through, about 5–7 minutes. Stir in the cooked quinoa or rice, diced tomatoes, olives, and feta cheese. Mix well and remove from heat.

Stuff the peppers:
Spoon the turkey mixture into the prepared bell peppers, pressing it down gently to pack the filling in. Place the stuffed peppers into the baking dish and cover with aluminum foil.

Bake:
Bake for 25–30 minutes, or until the peppers are tender and the filling is heated through.

Make the tzatziki sauce:
While the peppers are baking, prepare the tzatziki sauce. In a bowl, mix together Greek yogurt, grated cucumber, olive oil, lemon juice, garlic, dill, salt, and pepper. Stir until smooth and well combined.

Serve:
Once the peppers are cooked, remove from the oven and top each stuffed pepper with a generous spoonful of tzatziki sauce. Serve warm and enjoy!