Peel and finely chop the onion. Knead minced meat, onion, breadcrumbs, egg yolk, salt, pepper and mustard. Form small balls with moistened hands. Wash, clean and slice the tomatoes.
Clean, wash and quarter the mushrooms. Fry the meatballs in a pan with hot oil all around. Fry the mushrooms briefly. Add tomatoes and tomato paste, braise briefly and deglaze with 200 ml water.
Season with salt and pepper and braise for about 5 minutes. In the meantime clean and wash the chicory and cut it in half lengthwise. Bring salted water and lemon juice to the boil in a small pot and cook the chicory for about 5 minutes.
Drain it. Wash the chives, dab dry and cut into small rolls. Arrange the chicory on a plate with the vegetable mince mixture. Serve sprinkled with chives.