Preheat oven to 375°F (190°C).

Heat olive oil in a medium skillet over medium heat, add chopped onion and cook for 4 minutes until soft. Stir in minced garlic and cook for 1 additional minute.

Slice tops off bell peppers and remove seeds. Stand peppers upright in a 9x12-inch baking dish.

In a large bowl, combine onion mixture, chopped dill, salt, ground allspice, and black pepper. Mix in cooked rice and ground lamb, then gently fold in crumbled feta cheese.

Stuff peppers with the mixture, compacting it gently to fill each pepper fully.

In a bowl, mix tomato sauce with cold water, fresh lemon juice, and white sugar. Pour half of the mixture over the peppers and the other half into the bottom of the baking dish. Cover with aluminum foil.

Bake in the preheated oven, covered, for 45 minutes. Remove the foil and continue baking for an additional 15 minutes, occasionally basting the peppers with the sauce.