Cook fettuccine in salted boiling water according to package directions until al dente. Reserve one cup of pasta water before draining completely. The starchy pasta water helps bind the sauce later for perfect consistency.
Heat olive oil and butter in large skillet over medium-high heat until shimmering. Add torn oyster mushrooms in single layer, cooking undisturbed for 3-4 minutes. This allows proper browning and develops deep, earthy flavors throughout the dish.
Remove mushrooms and add diced onions to same pan with remaining fat. Cook until translucent, about 5 minutes, then add minced garlic. Sauté for another minute until fragrant, being careful not to burn the delicate garlic pieces.
Pour white wine into pan, scraping up any browned bits from bottom. Let wine reduce by half, then slowly add heavy cream while stirring constantly. This prevents the cream from curdling and creates a smooth, luxurious sauce base.
Return mushrooms to pan and add cooked pasta with fresh thyme. Toss everything together, adding pasta water gradually until sauce coats noodles perfectly. Remove from heat before adding grated Parmesan and fresh parsley for finishing touches