Using a food processor, puree the onion, bell pepper, cilantro and garlic.
At one end of each plantain on the concave side, make a V-shaped incision; wrap with a damp paper towel. Microwave at high heat for about 10 minutes. Check for doneness; the flesh should look cooked, not starchy. Otherwise, cook in 30-second increments until done.
Meanwhile, in a large skillet, heat the olive oil and crushed red pepper over medium-high heat. Stir in the onion mixture and tomato sauce and simmer for 2 minutes. Stir in the shrimp, season with salt and black pepper and cook until opaque, about 5 minutes.
Peel off the flap from each plantain. Halve lengthwise, being careful not to cut through to the other side. Push aside the 2 halves, forming a trench. Spoon in the shrimp mixture and serve.