Peel and thinly slice the onions and garlic. Sauté them in a mixture of oil and butter. Season with pepper, thyme, and salt.

Meanwhile, heat the chicken broth.

When the onions are well caramelized, deglaze with the balsamic vinegar.

Preheat the oven to broil (about 570°F/300°C).

Add the pasta to the onions and toast for about 3 minutes, stirring frequently.

Add the broth and cook the pasta according to the package instructions (about 11 minutes for me). Add the crème fraîche and mix well.

Transfer the mixture to an oven-safe dish, sprinkle with the shredded cheeses, and broil until golden and bubbly.

Serve hot.