Onion and garlic: Peel and dice the onion, finely chop the garlic. Set aside for later. Mushrooms: Brush off any dirt. Slice the mushrooms, but not too thinly. Set aside for later. Vegetable stock: I use stock cubes, so I just boil 500ml of water and add 1 stock cube to it. My stock cubes are 1 cube/500ml. Other: Boil a large pot of water. Finely chop the parsley.
Once the water is boiling, add in the pasta. Make sure the tagliatelle is vegan, or use any other pasta (farfalle is my next favourite pasta to use). Cook the pasta according to the package instructions. While the pasta is cooking, start with the sauce.
Heat up a little oil or water in a large pan and add the onion and garlic. Sauté for about 3 minutes or until the onion is translucent. Then add in the mushrooms. Add in a pinch of salt and pepper, the salt will draw out the moisture from the mushrooms and speed up the process. Sauté until the mushrooms are soft. Add a splash of water if the ingredients stick to the bottom of the pan. When the mushrooms are soft after about 3-5 minutes, add in the flour and cook the flour for about 2 minutes. Deglaze the pan with the vegetable stock and the vegan cream alternative. I use Alpro cuisine (soya), but there's also rice and oat creamer! Stir through until the flour is completely incorporated in the sauce. Leave the sauce to thicken for about 5 minutes, but stir occasionally.
At this point both the pasta and the sauce should be done. Drain the pasta and add to the sauce and stir through until all of the pasta is covered in sauce. The sauce will thicken more while you're doing this, so don't worry if you think the sauce is too runny. Turn the heat to the lowest setting and add in the frozen peas and the parsley. Stir through for about 1 minute more and then you're done!