Preheat the oven to 325°F and bring a large pot of salted water to a boil.
While the oven is heating up, prep all the ingredients and get them all in place to work with. This will greatly reduce the cooking time and help with timing.
When the oven is preheated, place the walnut pieces onto a baking sheet and toast in the oven for about 15 minutes. After 7 - 8 minutes, give them a toss and be careful not to let them overcook or burn. When done, remove from oven and let cool.
While the walnuts are toasting, heat up a fry pan over medium-high heat and when hot, add the butter. As soon as the butter melts, add the shredded sage leaves and cook for 2 - 3 minutes. You are making sage butter, one of flavors of all time.
At this point, the butter will start to brown which is good, but don't let it burn. Add lemon zest, cream, a little bit of salt and a couple of grinds of fresh pepper. Stir to combine ingredients.
Remove the pan from heat and set it aside until you are ready to add the pasta and other ingredients. Be careful not to let the cream separate by cooking too long.
When the pasta comes to a boil, add the tagliatelle and cook until al dente. This takes 8 - 12 minutes. When perfectly cooked, remove and reserve some of the pasta water in case you need it to thin out the sauce a little.
Reheat the sauce on medium-low and if too thick add some of the reserved pasta water.
You can drain now drain the tagliatelle and add it to a big bowl or if the pan you are using is big enough, add the pasta right to the sauce. Either way, add the walnuts, Parmesan and parsley and stir to combine.
Add the lemon juice, toss again, taste and adjust seasoning with salt and pepper.
I like to have some Parmesan on the table for those who want more.