Bring a large pot of water to a boil with a large pinch of salt. Halve, peel, and finely chop the red onion. Mince or grate the garlic. Finely chop the parsley and tarragon. Halve the lemon. Add the pasta to the boiling water and cook for about 10 minutes, until al dente. Be sure to reserve 1/2 cup pasta water.

Meanwhile, heat 1 tablespoon olive oil in a large pan over medium heat. Add the onion and season with salt and pepper. Cook for about 5 minutes, until softened. Add the garlic and mushrooms and season with salt and pepper. Cook for about 5 minutes, until mushrooms are golden brown.

Add the tarragon, stock concentrate, and 1/2 cup pasta water to the pan with the mushrooms. Cook for about 3 minutes, until slightly reduced.

Add the sour cream stir to combine, and cook for 3 minutes.

Drain the pasta and add it to the pan with the sauce. Add a squeeze of lemon juice and half the parsley. Toss to combine.

Serve the fettucine with a sprinkle of parmesan and remaining parsley on top.