Bring 5 cups of water to a boil in a large saucepan. Stir in lentils and bouillon cubes. Simmer over medium-low heat until lentils are nearly tender, 15 to 20 minutes.
Trim off the edges of nopales and scrape spines from both sides using a sharp knife. Rinse and roughly chop. Bring a pot of water to a boil. Add nopales and cook until tender, 8 to 10 minutes. Drain and rinse.
Heat a skillet over medium-low heat. Add pasilla peppers and toast until evenly brown, making sure not to burn them, about 5 minutes. Cool until safe to handle; remove seeds and veins. Combine toasted peppers, onion, garlic, and remaining 1 cup water in a blender. Process until smooth; strain into a bowl.
Stir blended pepper mixture and boiled nopales into the saucepan with the lentils. Simmer until lentils are tender, about 20 minutes more.