Drain shrimp, chop into ½-inch pieces and combine with lime juice.
Melt 2 tablespoons of butter in a skillet over medium heat. Sauté onions and garlic. Remove from heat and stir in jalapeno pepper and shrimp; set aside.
Spread butter on one side of tortillas and place butter side down on large rimmed baking sheet. Spoon about 2 tablespoons of cheese and 2 tablespoons of shrimp mixture (drain off liquid if necessary) on half of each tortilla. Fold tortillas in half over filling, so buttered side is out.
Preheat oven to 250°F.
In a large heavy skillet over medium heat, toast folded tortillas until golden – about 1 minute per side. Work in batches and transfer cooked tortillas to a baking pan in preheated oven to keep them warm. Use a pizza wheel to cut each folded tortilla into halves. Serve with salsa.