Wash and dry all produce. Bring a large pot of salted water to a boil. Halve, peel, and finely chop onion. Mince or grate garlic. Pick leaves from tarragon; discard stems. Finely chop leaves. Halve lemon. Thinly slice mushrooms.

Once water is boiling, add tagliatelle to pot. Cook until al dente, 9-11 minutes. Drain, reserving 1/2 cup cooking water.

Meanwhile, heat a drizzle of olive oil in a large pan over medium heat. Add onion and season with salt and pepper. Cook, tossing, until softened, about 5 minutes. Add garlic and mushrooms. Season with salt and pepper. Cook, tossing, until mushrooms are tender, about 5 minutes more.

Add stock concentrates, pasta cooking water, and half the tarragon to pan with mushrooms. Let simmer until slightly reduced, about 3 minutes. Stir in sour cream. Cook until slightly thickened, about 3 minutes more.

Add tagliatelle, a squeeze of lemon, and half the Parmesan to pan with sauce and toss to combine. Season with salt and pepper.

Divide tagliatelle and mushrooms between plates. Sprinkle with remaining Parmesan and tarragon (to taste) and serve.