Special equipment: a deep-frying thermometer
1. Preheat the oven to 250 degrees F. Line one baking sheet with a kitchen towel and another baking sheet with a cooling rack.
2. Combine 4 cups of water, 2 tablespoons salt and the juice of 1 lime in a large bowl. Add the plantains and let sit for 15 minutes.
3. Pour about 2 inches of canola oil into a large Dutch oven or heavy-bottomed pot. Heat over medium-high heat until a deep-frying thermometer inserted in the oil reaches 350 degrees F.
4. Transfer the plantains to the towel-lined baking sheet using a slotted spoon; blot dry with paper towels. Fry in batches until the outsides are light golden brown and the centers are tender, 6 to 8 minutes. Remove to the rack-lined baking sheet using the slotted spoon.
5. When the plantains are cool enough to handle (after 2 to 3 minutes), use the bottom of a can to flatten each plantain to 1/8 inch thick. Check to make sure that the canola oil is still at 350 degrees F. Fry the flattened plantains in batches until crispy and golden brown all over, 3 to 4 minutes. Remove to the rack-lined baking sheet with the slotted spoon. Generously salt the fried plantains and transfer the baking sheet to the oven to keep warm.
6. Heat the olive oil over medium-high heat in a large skillet. Add the onions, Cubanelles and red bell peppers and cook, stirring constantly, 2 minutes. Add the garlic and cook, stirring, another 2 minutes. Add the shrimp and oregano and sprinkle liberally with salt and pepper. Cook, stirring constantly, until the shrimp turn opaque and are just cooked through, 3 to 4 minutes. Turn off the heat and stir in the remaining lime juice and the chopped cilantro.
7. To serve, top the tostones with a portion of sofrito and shrimp. Garnish with additional cilantro leaves and serve with lime wedges.