Bring a large pot of salted water to a boil. Add the fettuccine and cook according to package instructions until al dente. Drain and set aside.

In a skillet, heat olive oil over medium heat. Add sliced onions and cook, stirring occasionally, for about 20-25 minutes until they are golden brown and caramelized.

Add minced garlic to the skillet with the onions and cook for an additional 1-2 minutes until fragrant.

Pour in the heavy cream and bring to a simmer. Stir in the grated parmesan cheese, salt, pepper, and thyme. Cook for 3-5 minutes until the sauce thickens slightly.

Add the cooked pasta to the skillet and toss to coat with the sauce. Adjust seasoning if necessary.

Serve warm, garnished with additional parmesan cheese and fresh thyme if desired.