Snap off woody ends of asparagus; cut each stalk crosswise diagonally into 1-inch pieces.
In a large skillet, heat oil over medium-high heat. Add garlic and red pepper flakes; sauté 1 minute. Add asparagus and sauté 2 minutes. Stir in hot water and 1/4 teaspoon salt; cover skillet and cook asparagus 2 minutes or until almost tender. Uncover and cook 1 minute longer. Remove from heat and set aside.
Cook fettuccine according to package instructions. Meanwhile, place salmon in a microwave-safe bowl and sprinkle with lemon juice and 1/8 teaspoon salt. Microwave 2 to 3 minutes or until fish is just cooked through. Separate fillet into bite-size chunks, and mix gently with cooking juices in bowl.
Drain pasta and mix with asparagus in skillet; add salmon with juices, gently toss and serve.