Rinse the onions with boiling water and drain.
In a large, 1 L Mason jar, mix the salt, sugar, vinegar, water and hot pepper flakes. Add the rinsed onions and marinate at room temperature for at least 30 minutes.
Mix the tuna and lime juice in a bowl, using a fork. Salt and pepper to taste, and adjust seasoning as desired.
Distribute the tuna mixture evenly on the 4 tostadas. Garnish with cherry tomatoes, avocado, marinated onions and fresh cilantro. Add pepper and serve with lime quarters and hot sauce.