Preheat your oven to 375°F (190°C).
Carefully cut the tops off the bell peppers and remove the seeds and membranes. Set aside the tops.
In a large skillet, heat the olive oil over medium heat. Sauté the onion and garlic until translucent, about 3 minutes.
Add the green lentils to the skillet and cook for another 2 minutes, stirring frequently.
Mix in the diced tomato, cumin, paprika, black pepper, and salt. Cook for 5 minutes, allowing the flavors to blend.
Stir in the chopped spinach and cook until it wilts, about 2 minutes.
Remove from heat and let the mixture cool slightly. Stir in the pine nuts (if using), parsley, and crumbled feta cheese.
Fill each bell pepper with the lentil and spinach mixture. Place the stuffed peppers in a baking dish.
Pour the vegetable broth into the bottom of the dish. This will help steam the peppers as they bake.
Replace the tops of the peppers. Cover the baking dish with foil and bake for 30 minutes.
Remove the foil and bake for an additional 10 minutes, or until the peppers are tender and the tops are slightly charred.
Serve the stuffed peppers warm, garnished with extra parsley if desired.