Heat the oven to 150 C. Onions are cleaned and cut into thin slices or crescents. In a large skillet getting hot 1 tbsp olive oil and 25 g butter. Add onion slices and sauté with the lid on low heat for 20 minutes. Onions must reduce its volume twice and become soft. Sprinkle with sugar, salt and ground black pepper. Thyme added and stirred. Onions are transferred in a fire pan and capped. Cook in the oven for 30-45 minutes to get quite soft, but not brown onions. The tray is removed from the oven and othlupva. Pour balsamic vinegar and onion mix. While stew onions, potatoes are boiled in salted water until tender. Drained of water and dried on low heat. Add 50 g butter, milk and 1 tbsp mustard. Season with salt and pepper and potato smash. Calf liver cut into slices. Place for 30-60 minutes in the milk to soak in water well. In a pan becomes heated remaining olive oil and butter. Add the mustard and sage leaves. Add drained liver slices and fry each side for 1-2 minutes. To the liquid obtained was added with frying lemon juice and mix. Veal liver served with onions, mashed potatoes and topped with the sauce from the pan.