100ml extra virgin olive oil
6 cloves garlic, finely sliced
7 sprigs thyme
1 celery heart, finely sliced
6 large Spanish red onions, cut into 1cm-thick slices
6 brown onions, cut into 1cm-thick slices
1.5L beef stock
100ml dry sherry
Salt and freshly ground pepper, to season
200g Gruyère, sliced
1 baguette, cut into thick slices
½ bunch flat-leaf parsley leaves picked, very finely chopped
Combine oil, butter, garlic, thyme, celery, ½ the Spanish onion and ½ the brown onion in a large saucepan over a low heat. Cook slowly for about 45 minutes or until mixture caramelises.
Add remaining onion to pan and cook for a further 20 minutes. Stir in stock and simmer gently for another 20 minutes. Stir in sherry and season with salt and pepper.
Preheat grill to medium. Put 1 slice of cheese on each bread slice. Grill until bread is toasted and cheese is golden and bubbling.
Ladle soup into bowls. Top with 1 or 2 cheese-covered bread slices. Sprinkle parsley over bread and serve.