Wash the porcini mushrooms and cut into thin slices. Saute them in a skillet with a knob of butter and 1 minced garlic clove. Add a sprinkling of chopped parsley, and let cook for about 10 minutes over low heat. Meanwhile, cook the spaghetti to al dente, adding the asparagus a few minutes before the pasta is done. Once cooked, drain the pasta and asparagus. Combine with the porcini mushrooms. Season with salt and pepper to taste, and serve immediately.