Bring a 6-quart / 5.7L pot of salted water to a boil. Add the tagliatelle and cook according to package directions, about 8 minutes, until al dente. Drain, reserving 1 cup of the pasta water.

Heat 1 tbsp olive oil and 15 g butter in a 12-inch / 30cm skillet over medium-high heat. Add half the cremini and shiitake mushrooms in a single layer and cook for 5 to 6 minutes, stirring occasionally, until deeply browned. Transfer to a bowl and repeat with the remaining mushrooms and the oyster mushrooms, adding the rest of the oil and butter as needed.

Return all the mushrooms to the skillet. Add the shallots and cook for 2 minutes until softened. Add the garlic and cook for 30 seconds until fragrant.

Pour in the brandy and scrape the bottom of the pan to loosen any browned bits. Let it bubble for 1 minute until mostly evaporated.

Add the vegetable stock and thyme, bring to a simmer, and cook for 3 to 4 minutes until slightly reduced.

Lower the heat, stir in the sour cream and Dijon mustard, and simmer gently for 2 minutes until the sauce coats the back of a spoon. Do not let it boil or the sour cream can split.

Add the drained tagliatelle to the skillet and toss to coat, loosening with a splash of the reserved pasta water if the sauce looks tight.

Season with salt and black pepper to taste. Divide between bowls, top with parsley and parmesan if using, and serve right away.