Peel and thinly slice onions
Cook onions slowly in butter for around 30 minutes to caramelize. Stir often to obtain a uniformly light brown caramelized color. Keep stiring and cooking until color changes.
Season with salt and pepper
Sprinkle with flour, then continue to cook until the flour is well browned.
Deglaze with a dash of white wine.
Add stock and simmer for about 30 minutes.
Pre-heat broiler in the oven. Cut baguette into slices. Toast the slices on both sides.
Add cheese and set under the broiler until golden. Set aside.
Serve soup warm topped with bread-cheese slices.