Step 1Heat the oven to 425 degrees.
Step 2Toast the oregano in a dry skillet over medium-low heat until fragrant, 30 seconds.
Step 3Toss together the oregano, serrano chile, cilantro, onion, 2 tablespoons of lime juice and the shrimp. Let the shrimp marinate while you prepare the remaining ingredients.
Step 4Cut the avocado in half, remove the pit and scoop the avocado out of the shell into a small bowl. Mash it with a fork, then stir in the remaining 1 teaspoon of lime juice. Season the guacamole with salt and pepper to taste and set aside.
Step 5Lightly spray both sides of the tortillas with nonstick cooking spray and place them in a single layer on a baking sheet. Bake until lightly browned and crisp, 10 to 12 minutes. Remove them from the oven and allow to cool slightly so you can handle them.
Step 6When ready to serve, divide the guacamole among the baked tortillas, spreading it evenly over each. Sprinkle about 2 tablespoons of the black beans over the top of each tostada, then top with the romaine and radish slices. Spoon the shrimp, along with the marinade, over the top of each and top with thin slices of red pepper. Garnish each plate with a radish and 2 lime wedges.