Shrimp and Scallop Aguachile

Using a sharp knife, cut shrimp in half to create two thinner halves. Very thinly slice scallops; transfer shrimp and scallops to a large platter. Thinly slice 3/4 of cucumber.
Add remaining ¼ of the cucumber, ½ cup cilantro, ½ cup lime juice, 2 cloves garlic, ¼ cup red onion, 1 serrano, and ¼ teaspoon salt to a blender. Blend until smooth, about 1 minute.
Drizzle sauce over shrimp and scallops and lightly toss to combine.
Arrange cucumber around perimeter of platter. Sprinkle remaining ¼ cup red onion over seafood then arrange sliced avocado over top and sprinkle with some cilantro leaves.
Break tostadas into large chip-size pieces and serve with aguachile immediately.
Note: The longer you let agauachile sit, the more *cooked* the fish will get. Aguachile is meant to be served like raw sashimi. If you prefer a ceviche-like experience, let the dish sit for 5–20 minutes before serving.