Lay out crackers on a serving platter. Melt butter in a large skillet over medium-low heat. Add shrimp; season with garlic salt. Cook and stir until pink all the way through, 3 to 5 minutes per side. Place 1 shrimp on each cracker. Heat olive oil in a separate skillet over medium heat. Add fennel, garlic, and thyme; cook and stir until fennel softens, about 15 minutes. Remove from heat. Season fennel with salt and pepper. Stir in basil. Spoon fennel mixture evenly over shrimp.