Place a rack in the middle of the oven and preheat it to 220 °C / 430 °F. Line your baking sheet with parchment paper. Cut the bread into slices, then cut into large chunks. Arrange in a single layer over the baking sheet and place in the preheated oven. Bake for 8 minutes at 220 °C / 430 °F.
Peel the onion and cut it into thin slices. Place a large pot over medium-high heat. Add the butter and oil. When the butter melts, add the onion. While stirring, cook the onion for 10 minutes. Season with salt and add thyme. Cook for 20 - 30 minutes on low heat or until golden brown. While cooking, stir the onion regularly, especially in the bottom of the pan. Add the wine, cook for 5 minutes for the alcohol to evaporate, then add the stock and stir to combine. Simmer for 10 minutes.
Transfer two ladles of the soup to a blender or a bowl if you're using an immersion blender. Mix into a smooth soup and add back to the pot with the soup. Check the seasoning and season with salt and pepper, if necessary. Discard the sprig of thyme. Transfer the soup to a small castiron or oven-safe pot or into small one-serving individual oven-safe pots. Arrange the toasted bread on top and sprinkle with a generous amount of cheese. Place back in the oven and bake for 8 - 10 minutes at 210 °C / 410 °F, settings on low broil.TipPlace the pot or individual pots on a baking sheet to prevent leakage of the soup in your oven.
When the cheese melts and is bubbling, remove from the oven and set aside for a couple of minutes. Serve with chopped chives.