Preheat oven (electric cooker: 175°C/circulating air: 150°C/gas: level 2). Dab fillet dry. Heat oil in a pan. Brown the fillet for 4-5 minutes. Season with salt and pepper. Place in an ovenproof dish.
Place on the grid of the hot oven and roast for 12-15 minutes. Keep the meat in the pan.
In the meantime, cut the baguette into slices, place on the oven rack and roast for 3-5 minutes.
Onions peel, halve and cut into strips. Heat the gravy. Brown the onions in it. Sprinkle with sugar, caramelise for 1-2 minutes while stirring. Deglaze with 3-4 tbsp. water and vinegar, bring to the boil and simmer for 2-3 minutes.
Wash thyme, shake dry, chop and stir in.
Let the fillet rest in aluminium foil for about 5 minutes. Cut cheese into slices. Cut fillet into slices, pour gravy over the onions. Cover baguette with fillet and cheese. Add onion sauce.