For the shrimp:
Heat the olive oil in a large frying pan on medium-high heat. Sauté the onion and bell pepper for 3-5 minutes or until the onion is translucent. Add shrimp and cook them until they turn pink. Add chopped tomatoes, serrano pepper, the blended mixture and fresh cilantro; season with salt, to taste. Reduce heat as soon as it starts to boil, cover and let cook for 10-12 minutes, until the sauce thickens a little more.