Shrimp Ceviche Cocktail with Avocado Crema and Fresh Tostadas

Name: Shrimp Ceviche Cocktail with Avocado Crema and Fresh Tostadas
Prep time: 20 min
Cook time: 5 min
Total time: 25 min
Cuisine: Latin American, American, Mexican
Ingredients: 1 cup clam juice; 1 pound raw large shrimp, peeled and deveined; 1/4 teaspoon dried oregano; 1 cup diced English cucumber; 1/2 cup diced celery; 1/2 cup finely diced red onion; 2 tablespoons jalapeno peppers, seeded, finely diced; 1 1/4 cup diced, peeled, seeded ripe tomatoes, or quartered small cherry tomatoes; 1/2 cup ketchup; 2 freshly squeezed limes; Freshly chopped cilantro; Mexican hot sauce to taste; 1 cup Masa Harina; 1 cup Water; Tortilla press or rolling pin; Wax paper; Vegetable oil; 2 ripe avocados, halved, pitted and meat scooped out; 1/2 cup Mexican crema; 1 clove garlic; 1/4 cup freshly squeezed lime juice; 1/2 teaspoon kosher or coarse sea salt, or to taste