Clean and wash the Brussels sprouts. Peel and wash potatoes and celery and cut them into large cubes. Cook the potato and celery cubes in boiling salted water at medium heat for about 20 minutes.
Cook the Brussels sprouts in boiling salted water for 12-15 minutes until al dente. Wash the meat, dab dry and cut in half. Wash rosemary, dab dry and halve twigs. Tie the steaks with kitchen twine into shape, tying 2 half twigs of rosemary diagonally into each.
Season with salt and pepper. Heat 2 tablespoons of oil in a coated pan. Brown the meat in it all around for 3-4 minutes. Place the meat on a baking tray, pour the roasting set over it and cook in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 7-8 minutes until medium heat.
Peel onions and garlic and cut into pieces. Heat 2 tablespoons of oil in a frying pan. Fry the onions in it for 3-4 minutes while tossing. Wash the thyme, shake dry and pluck the leaves from the stems.
Push the onions to the edge with the spatula. Add the sugar in the middle and let it caramelise until golden brown. Deglaze with balsamic vinegar and red wine, add thyme and garlic, bring to the boil and reduce briefly to a sauce-like consistency.
Season to taste with a little salt and possibly sugar. Drain the Brussels sprouts. Melt 25 g butter in a pot. Swivel the sprouts in it. Season with salt and pepper and keep warm. Drain potatoes and celery.
Put milk in a pot, season with salt, nutmeg and pepper, heat up and melt 50 g butter in it. Add potatoes and celery and mash to puree. Remove from heat. If necessary, heat onions again.
Arrange puree, Brussels sprouts, meat and onions on preheated plates.