Stir Augusto Black Truffle Sauce into cream. Cover and chill 6 hours.
Melt butter in a 12″ skillet over medium-high. Cook mushrooms and garlic until golden, 6-8 minutes. Add wine and thyme; cook 8 minutes. Add reserved truffle cream; simmer until thickened, 10-12 minutes. Cook tagliatelle in salted boiling water until al dente, about 7 minutes. Drain pasta; add to skillet. Add parmesan, lavender, salt, and pepper; toss to coat.
Garnish with watercress and Pecorino Romano; shave truffle over the top.