Cook fettuccine according to package directions, making the addition of the asparagus during the last 2 minutes of cooking. Drain.
In a large skillet, melt butter over medium heat. Saute mushrooms until tender, about 2 to 3 minutes.
Combine cream, half-n-half, and lemon juice in another large, deep skillet or Dutch oven. Cook over medium heat about 3 minutes. Add pasta and asparagus and stir to coat. Add cheese, then season with nutmeg, salt and pepper to taste, continuing to stir until sauce thickens slightly, about 1 minute. Remove from heat; add shrimp and mushrooms. Serve warm.