Step 1: Begin by preparing your shrimp. Ensure it is peeled, deveined, and chopped into small, bite-sized pieces.
Step 2: Place the chopped shrimp in a non-reactive bowl. Pour the fresh lime juice over the shrimp, making sure all pieces are fully submerged. Cover the bowl and refrigerate for 30-45 minutes, or until the shrimp turns opaque and pink throughout.
Step 3: Once the shrimp is 'cooked', carefully drain most of the lime juice from the bowl. In a large mixing bowl, combine the marinated shrimp with the diced tomato, finely chopped red onion, diced cucumber, chopped jalapeño, and fresh cilantro. Gently toss to mix all ingredients evenly.
Step 4: Season the ceviche mixture with salt to taste. Remember to taste and adjust, as the lime juice already provides a good amount of flavor.
Step 5: Just before serving, gently fold in the diced avocado. This prevents it from browning and keeps its creamy texture intact.
Step 6: Generously pile the vibrant ceviche mixture onto crispy tostadas. Serve immediately and garnish with any optional toppings like a drizzle of Mexican crema, a dash of hot sauce, or extra lime wedges for an added burst of flavor.