Preheat oven to 350 degrees F (175 degrees C). Arrange bread slices on a baking sheet.
Toast bread in the preheated oven until lightly golden, about 5 minutes.
Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned but still pliable, about 8 minutes. Drain bacon slices on paper towels.
Bring a large pot of lightly salted water to a boil. Add asparagus and cook uncovered until bright green, about 2 minutes. Drain in a colander and immediately immerse in ice water for several minutes to stop the cooking process. Drain.
Melt butter in a skillet over medium heat; cook and stir asparagus in hot butter under tender, about 5 minutes. Cut asparagus into pieces that will fit the bread slices. Season with garlic salt; wrap with bacon slices.
Arrange bacon-wrapped asparagus on toasted bread slices. Cover with mozzarella cheese slices. Sprinkle red pepper flakes on top.
Bake in the preheated oven until mozzarella cheese is melted and lightly browned, 3 to 5 minutes.