Shrimp with Ancho Butter Sauce

Name: Shrimp with Ancho Butter Sauce
Ingredients: 3 chiles de árbol; 3 dried ancho chiles; ½ small onion, unpeeled, halved; 2 garlic cloves, unpeeled; 2 Tbsp. fish sauce (such as nam pla or nuoc nam); 1 Tbsp. light brown sugar; Kosher salt; 2 lb. large fresh head-on shrimp (about 16); 4 Tbsp. olive oil, divided; 4 garlic cloves, thinly sliced; 1 Tbsp. heavy cream; ¾ cup (1½ sticks) unsalted butter, cut into pieces; 3 Tbsp. (or more) fresh lime juice; Kosher salt and freshly ground black pepper; Finely chopped fresh flat-leaf parsley and crackers or toasted country-style bread (for serving)