Preheat your oven to 400°F (200°C) and line a baking sheet with parchment for easy cleanup.
Slice the tops off the peppers and carefully remove the seeds and membranes. If needed, trim the bottoms slightly so they stand upright. Set aside.
In a large skillet, heat olive oil over medium heat. Add the minced garlic and sauté for about 30 seconds until fragrant.
Add the ground chicken to the skillet. Break it up with a spatula and cook until it’s no longer pink, about 5–7 minutes.
Stir in the dried oregano, cumin, and smoked paprika. Let the spices bloom for 1 minute, then add the diced tomatoes and cooked quinoa. Cook for 2–3 minutes to let the flavors mingle.
Remove the pan from heat and fold in the crumbled feta and chopped parsley. Taste and season with salt and pepper as needed.
Spoon the filling into each prepared pepper until they are generously packed. Place the stuffed peppers on the lined baking sheet.
Sprinkle the shredded mozzarella on top of each pepper. This will melt into a creamy, golden crust as they bake.
Bake for 18–22 minutes, or until the peppers are tender and the cheese is bubbling and lightly browned.
Remove from the oven and let the peppers rest for 5 minutes before serving. Garnish with extra parsley if you like.