1. Melt butter in a large skillet over medium-high heat. Add shallots, thyme, salt, and mushrooms; sauté 4 minutes. Add stock. Bring to a boil; reduce heat, and simmer until liquid is reduced by half (about 2 minutes). Remove from heat.
2. Bring 12 cups water to a boil in a Dutch oven. Shave asparagus into thin strips using a vegetable peeler. Add pasta to pan; cook 2 minutes. Add asparagus; cook 2 minutes. Drain.
3. Return mushroom mixture to medium-high heat; add half-and-half and lemon rind. Add cream cheese and pepper, stirring with a whisk until smooth. Add mushroom mixture to pasta mixture in Dutch oven; toss to combine. Top with tarragon.