Make the dressing: In a medium bowl, mix together the garlic, serrano, lime juice, mayonnaise, ½ teaspoon salt and ½ teaspoon pepper until well combined; set aside.
Make the salad: Roughly chop the cilantro, chop the avocado, dice the red onion and tomato, and place everything in the bowl with the dressing. Add the tuna and ¼ teaspoon salt and toss until combined. Taste and add more salt as desired. Serve immediately, or cover and refrigerate for up to two days.
When you are ready to eat, divide the filling among the tostada shells (about a generous ½ cup tuna salad for each tostada) and top with hot sauce, if desired.