Bring water to a rolling boil in a very large pot. The larger, the better in order to give spaghetti noodles a chance to dance around while cooking.
Add kosher or sea salt to the water. Add spaghetti noodles. Gently swirl the noodles once with a wooden spoon. Bring noodles to a gentle boil, then turn heat down just a bit. Place fresh asparagus in a metal strainer directly above the cooking spaghetti. Cover with a lid. Gently swirl the pot every 3-4 minutes until spaghetti is al dente and asparagus is slightly soft and bright in color.
Remove asparagus, drizzle with 1 Tbsp of your butter and a pinch of your salt. Cover and set aside. Drain spaghetti, reserving 1 cup of pasta water. Return noodles to the original pot, gently stirring in the reserved pasta water. Add remaining butter to pasta. Serve spaghetti and asparagus side by side. You will find that the asparagus cooked above has given a nice flavor to the pasta.