In a medium saucepan stir together water, lentils, onion, salt, cumin, and garlic. 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; stir in cilantro.
Spread lentil mixture on tostada shells; top with vegetables and cheese. Place on a large baking sheet. Broil 6 inches from the heat about 2 minutes or until cheese melts.