Peel and dice the onion. Place the pistachios in a dry pan on low/ medium heat and toast them until fragrant.
Bring a large pot of salted water to a boil and cook the tagliatelle until al dente (2 min less than suggested). Drain but leave some of the cooking water.
Now fry the onions in a large skillet (which can hold the pasta) on medium heat. Fry until soft then add half of the chopped pistachios and stir well. Add the double cream and cook for about 1-2min.
Add the tagliatelle to the skillet and stir/toss to coat the pasta in the cream sauce. Add the 3 tbsp of parmesan cheese, black pepper and a pinch of salt. Still well and if the sauce is too thick, add a little of the pasta water.
Divide and place the tagliatelle onto two plates, garnish with the remaining pistachios and extra parmesan cheese. Serve and enjoy.