Caramelise the Onions: Place your largest, deep-sided pot over a medium heat. Add the butter and olive oil, letting the butter melt and foam. Tip in the sliced onions and the teaspoon of salt. Stir well to coat. Cook for 20-25 minutes, stirring every 3-4 minutes. You'll see the onions turn from white to translucent, then slowly deepen to a rich golden brown. The smell will become sweet and intensely savoury. They should be soft and jammy, not crispy.

Add Aromatics: Once the onions are a deep amber colour, add the minced garlic and fresh thyme. Stir constantly for about 1 minute until you can smell the garlic becoming fragrant — be careful not to let it burn.

Make the Roux: Sprinkle the flour over the onions and stir vigorously. Cook for 2 minutes, stirring constantly. The mixture will become thick and pasty, and you'll see it pulling away from the bottom of the pot slightly.

Deglaze and Build the Broth: Slowly pour in the beef stock while whisking continuously to prevent lumps. Add the black pepper. Bring the mixture to a gentle boil; you'll see bubbles around the edges of the pot. The liquid will thicken slightly and turn a deep, glossy brown.

Cook the Pasta: Add the pasta to the pot, pushing it down so it's mostly submerged in the liquid. Bring the heat down to a steady simmer. Cook according to the pasta package instructions, usually 10-12 minutes, stirring occasionally with tongs to prevent sticking. The sauce will reduce and become thicker as the pasta absorbs the stock. The pasta should be tender but still have a slight bite (al dente).

Finish with Cheese: Remove the pot from the heat. Sprinkle the grated Gruyère and Parmesan over the top. Gently stir the pasta. The residual heat will melt the cheese into a creamy, luscious sauce that coats every strand. The sauce should be smooth and stretchy, not clumpy.

Serve Immediately: Divide the pasta among warm bowls. Garnish with a generous sprinkle of fresh parsley. Serve right away while the cheese is still wonderfully molten.