Steam cactus, covered, 5 minutes; cool. Chop to measure 1/2 cup; set aside. Combine egg substitute and salt in a small bowl, stir with a fork.
Heat oil in a large nonstick pan over medium heat; add onion and serrano, stirring frequently. Sauté 2 minutes; stir in egg mixture, stirring constantly to prevent egg from sticking to pan. Remove from pan when eggs are thoroughly cooked; keep warm.
Stack tortillas; wrap in damp paper towels and microwave on high for 25 seconds. Place one tortilla on each of 4 plates. Spoon 1/4 cup egg mixture over half of tortilla; top with 2 tablespoons cactus, 2 tablespoons cheese, 1 tablespoon tomato, 1 teaspoon Salsa de Tomatillo, and 1 teaspoon cilantro. Fold remaining tortilla half over filling; repeat with remaining tortillas and ingredients.
Note: Look for cactus paddles in the produce section of most supermarkets.