Caldo de Camarón

Name: Caldo de Camarón
Ingredients: ¼ cup grapeseed or canola oil; ½ lb dried shrimp; 1 lb head-on small shrimp, peeled (reserve the heads and shells); 1 large white onion, halved; 5 small garlic cloves, peeled; 2 Tbsp. dried Mexican oregano; 10 guajillo chiles, seeded; 2 dried Costeño chiles or chiles de árbol, seeded; 8 plum tomatoes; 1 large carrot, cut into small cubes; ½ cup fresh peas; ½ cup epazote leaves; Salt; ½ cup finely chopped white onion; 4 Tbsp. minced fresh cilantro; 3 serrano chiles, dices; Key lime wedges; 10 tostadas