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 and discard stems. Finely chop leaves. Halve lemon. Thinly slice mushrooms.

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

Meanwhile, heat a drizzle of oil in a large pan over medium heat. Add onion and season with salt and pepper. Cook until softened, about 5 minutes, tossing occasionally. Add garlic and mushrooms. Season with salt and pepper. Cook until mushrooms have shrunk and turned a few shades darker, about 5 minutes more.

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

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

Divide linguine and mushrooms between plates. Sprinkle remaining Parmesan and tarragon over each and serve.