Slowly caramelise the onions in the butter in a heavy based pan.
After 10 minutes add a pinch of salt, the garlic, and bay leaves.
Cook, stirring regularly, until the onions are caramelised and brown, about 45 minutes.
Pick the thyme leaves and add to the onions, along with the wine.
Bring to a simmer and cook until the wine is reduced by half.
Add the beef stock and soy sauce and gently simmer for around 10 minutes.
Meanwhile, slice the baguettes to 2cm thickness and grate the Gruyère cheese.
Check the seasoning, adding some salt and pepper if required.
Serve the hot soup in individual (oven-safe) bowls, top with sliced baguette and mound high with grated Gruyère.
Place under a hot grill until golden brown and bubbling.
Sprinkle with chopped chives.
