Cut 1 bunch asparagus into thirds. Heat 2 teaspoons sesame oil in a skillet over medium heat. Cut a 1-inch piece ginger into matchsticks, add to the skillet and cook, stirring, 1 minute. Add 3 cups quartered shiitake mushroom caps and 1 sliced garlic clove and cook, stirring, 2 minutes. Add the asparagus and 1/3 cup water; bring to a boil, cover and cook until crisp-tender, 3 minutes. Add 1 tablespoon each soy sauce and butter; cook, stirring, 1 minute. Season with salt and pepper.