Preheat your oven to 375°F (190°C). Slice the tops off the bell peppers and remove the seeds and internal membranes.
In a large skillet over medium heat, add the olive oil and sauté the diced yellow onion until translucent, about 4 minutes.
Add the minced garlic and ground turkey to the skillet, breaking the meat up with a spoon until browned and fully cooked through.
Stir in the cooked brown lentils, tomato puree, dried oregano, sea salt, and black pepper, simmering for 5 minutes to meld the flavors.
Place the bell peppers in a baking dish and spoon the turkey and lentil mixture into each cavity until they are tightly packed.
Pour 0.25 cup of water into the bottom of the baking dish, cover tightly with foil, and bake for 30 minutes.
Remove the foil, sprinkle the tops with crumbled feta cheese, and bake for an additional 5 to 10 minutes until the peppers are soft and the cheese is slightly melted.