Preheat the oven to 350°F (175°C) if you are baking your tostada shells to make them crispy. If using store-bought crispy tostadas, you can skip this step.
In a mixing bowl, combine the drained tuna, mayonnaise, Sriracha, lime juice, soy sauce, salt, and pepper. Stir well until the mixture is fully combined. Taste and adjust the spice level by adding more Sriracha if needed.
If baking the tostadas, place them on a baking tray and bake in the preheated oven for about 5 minutes or until they are crispy and lightly golden.
While the tostadas are baking, dice the ripe avocado and set it aside. You can add a pinch of salt and a squeeze of lime juice to the avocado to enhance the flavor and prevent browning.
Once the tostadas are ready, remove them from the oven and allow them to cool for a minute. Top each tostada with a generous spoonful of the spicy tuna mixture.
Add diced avocado on top of the spicy tuna for a creamy texture. Sprinkle with chopped red onion and cilantro for extra flavor and garnish.
Serve immediately and enjoy your Spicy Tuna and Avocado Tostadas!