Cook the lentils and vegetables: Add the lentils to a large pot. Place the whole potato, onion, and carrots on top. Cover with water, season lightly with salt, and cook until everything is soft (about 25–30 minutes).
Mash everything: Drain excess water if there is any. Mash the lentils, carrots, onion, and potato together in the pot until mostly smooth but still a little chunky.
Make the mixture: Transfer the mash to a bowl. Add breadcrumbs, chopped parsley, paprika, cumin, salt, pepper, and chili flakes (optional). Mix until the mixture holds together. Add more breadcrumbs if needed.
Shape patties: Form oval or square patties by pressing the mixture firmly with your hands.
Fry: Heat oil in a pan over medium heat. Fry patties on both sides until golden brown and crispy.
Assemble sandwiches: Slice the bread, add the sauce, tomato slices, onion-parsley mix, 3–4 lentil patties, and grilled green pepper. Press lightly and enjoy warm!
