Bring a large pot of salted water to a boil; begin cooking fettuccine noodles according to package.
Melt butter over medium-high heat in a large skillet. Add onion and cook for 2 minutes. Add garlic, red pepper flakes, and salt. Cook for an additional 2 minutes. Deglaze pan with broth/wine.
Add asparagus to skillet. Cook for 2 minutes and then add the tomatoes to the skillet. Toss and let cook for an additional 4 minutes or until asparagus is tender but still has a slight crunch. Add pasta water and transfer pasta to skillet. Toss pasta and vegetables. Serve immediately (excellent with shaved parmesan cheese and basil - or on its own!).