In a medium saucepan stir together water, lentils, onion, cilantro, garlic, salt, and cumin. Bring to boiling; reduce heat. Simmer, covered, for 12 to 15 minutes or until lentils are tender and most of the liquid is absorbed. Use a fork to mash the cooked lentils.
Spread the lentil mixture on tostada shells; top with the vegetables and cheese. Place on a large baking sheet. Broil tostadas 3 to 4 inches from the heat about 2 minutes or until cheese is melted.