Add black beans, water and three teaspoons of lime juice to food processor bowl and process until smooth. Mix Greek yogurt with spices and set aside. Chop onions and lettuce. Dice avocado; toss with remaining teaspoon of lime juice and salt to taste.
Set oven to broil. Place tortillas on baking sheet. Spread ~ 1/4 cup of black bean spread on tortilla and sprinkle with 1/8 cup of shredded cheese. Broil in oven until cheese is melted. Remove each tostada to a plate and top with 1/8 cup salsa, 1/2 cup lettuce, 1 T. green onions, and 1-2 T. spicy yogurt. Sprinkle tostada with avocado and hot sauce to taste.
Spice level is mild - medium, but may be adjusted for taste.