Shrimp Ceviche

Bring a large pot of salted water to a boil. Turn off heat, add shrimp, and poach until shrimp are pink and cooked through, about 3 minutes. Drain, let cool, then cut shrimp into bite-size pieces. In a large bowl, combine shrimp with lime juice, orange juice, onion, and jalapeños. Season with salt, then let marinate in the refrigerator, 1 hour. Stir in cucumber, tomatoes, and cilantro, and season with salt and pepper.  In a small bowl, mix together mayo and hot sauce. Spread spicy mayo onto tostadas, then add ceviche. Top each tostada with sliced avocado and serve immediately. Remove tails from shrimp and cut into bite-sized pieces. In a large bowl, combine shrimp with lime juice, orange juice, onion, and jalapeños. Season with salt, then let marinate in the refrigerator until shrimp is opaque and firm, 4 hours. Stir in cucumber, tomatoes, and cilantro, and season with salt and pepper.  In a small bowl, mix together mayo and hot sauce. Spread spicy mayo onto tostadas, then add ceviche. Top each tostada with sliced avocado and serve immediately.