In a large mixing bowl, combine the cooked lentils, chopped spinach, breadcrumbs, Parmesan cheese (if using), chopped onion, minced garlic, cumin, smoked paprika, salt, and black pepper. Mix well until all ingredients are evenly incorporated.
Add the egg to the mixture and stir until fully combined. If the mixture feels too wet, add a little more breadcrumbs until it holds together.
Divide the mixture into 4 equal portions and shape each portion into a patty.
Heat the olive oil in a large skillet over medium heat. Once hot, add the patties to the skillet, cooking for about 4-5 minutes on each side, or until golden brown and heated through.
Serve the burgers on buns with your favorite toppings, such as lettuce, tomato, avocado, or a dollop of yogurt.