Bring a large pot of water to a boil. Add salt and tagliatelle. Cook until al dente, according to package instructions.
In a medium skillet, melt butter over medium heat. Add onions, and cook, stirring, until translucent, about 3 minutes. Stir in 1 ounce truffles and a pinch of salt. Cook until tender, 2 to 3 minutes. Add Prosecco and cook until reduced by half, about 5 minutes.
Add cheese, and whisk until melted. Stir in remaining 1 ounce truffles. Cook for 2 minutes. If sauce is too thick, stir in cream. Stir in chives.
Drain tagliatelle, and place in a large bowl. Add sauce, and toss to combine.