Slice the onions into rings. Add a mix of sunflower and olive oil to a large pot. Season the onions with salt, pepper, and turmeric. Sauté the onions in the oil until they are soft and translucent.

Deglaze with 1 cup (250 ml) dry white wine. Add 4 1/4 cups water (1 liter) and chicken bouillon. Sprinkle in 3 or 4 heaping pinches of flour and stir well. (I like to add a bit of parsley.) Cover, bring to a boil, and simmer for 25–30 minutes. Turn off the heat and let rest for 10 minutes.

Use an oven-safe bowl, pour in the soup, add slices of bread (homemade is best), and sprinkle with shredded Gruyère cheese.

Bake in the oven for 5 minutes at 400–410°F (200–210°C) to melt the cheese. Enjoy with a good red wine (like a Médoc, for example).