Preheat an oven to 450 degrees F (230 degrees C). Grease a baking dish.
Combine millet, water, and vegetable bouillon in a saucepan; bring to a boil. Reduce heat to low, cover, and simmer until water is absorbed, about 15 minutes.
Heat olive oil in a skillet over medium heat; cook and stir onion and garlic until onion is tender, 8 to 10 minutes. Add ground beef and cook until beef is crumbly and browned, 8 to 10 minutes. Stir in tomato paste, paprika, cumin, cayenne pepper, and black pepper; stir and simmer to combine flavors, about 5 minutes.
Mix cooked millet, cilantro, and feta cheese into ground beef mixture. Spoon mixture into red bell peppers; place in prepared baking dish.
Bake in the preheated oven until peppers are slightly tender, 15 to 20 minutes. Place the tops back on the peppers; bake until peppers are tender, 10 to 15 more minutes.