Preheat your oven to 350°F (180°C). Heat the oil in a large frying pan over medium heat, add the onions, and cook gently for 3–5 minutes, stirring occasionally, until they soften and turn translucent without taking on any color. Add the garlic and diced green bell pepper to the pan and continue cooking, stirring from time to time, for 3 minutes or until the pepper becomes tender.
Sprinkle in the paprika, chili powder, and cumin, stirring continuously for 1 minute to toast the spices and release their aromatic oils. Add the tomatoes and cook for 3–5 minutes, stirring occasionally, until they break down and become soft and saucy.
Reduce the heat to low and gently fold in the shrimp and chopped cilantro. Season the mixture with salt and pepper to taste, stirring just until the shrimp are heated through. Be careful not to overcook the shrimp or they’ll become tough.
While the shrimp mixture is finishing, place the taco shells in the preheated oven and warm them for 3 minutes, or follow the timing specified on the package instructions, until they’re crispy and hot.
Remove the taco shells from the oven and fill each one generously with the warm shrimp mixture. Top with shredded lettuce for freshness and crunch, then garnish with sliced chiles, a whole shrimp or two for presentation, and extra cilantro sprigs. Serve immediately while everything is hot and the shells are still crispy.