Preheat your oven to 350˚F. Place your oven-proof soup bowls onto a baking tray lined with silicone so they do not slide around.
Heat the olive oil in a large dutch oven or stockpot set over medium high heat. Fry the onions for 15 -20 minutes, tossing frequently so they don’t burn. They will become translucent and darker in color. Add the flour and stir to coat the onions. Continue to fry for about 3 minutes, stirring frequently.
Add the white wine to the pot and stir well with a wooden spoon so there are no lumps in your mixture. Allow to simmer until the wine has reduced by 3/4. Add the stock and pepper. Stir to combine and bring the mixture to a gentle simmer. With the lid slightly ajar, leave to simmer for one hour, stirring every 15 minutes.
While the soup is cooking, slice your baguette into 1/2 inch slices that will fit into your serving bowls.
After an hour, taste the soup and add any salt or more pepper as you like. Ladle the soup into your serving bowls. Quickly dip a slice of baguette into the soup and then leave to float on the surface. Add as many baguette slices as will fit on the surface. Sprinkle with the grated cheese. Repeat with each serving bowl.
Transfer the tray of prepared bowls to the oven and bake for 15 minutes. Serve immediately and with care – the bowls are hot!
If you want to make this suitable for vegetarians, simply substitute the chicken stock with vegetable stock.