Wash the savoy cabbage and remove 8 outer leaves. Blanch the leaves in boiling salted water for about 5 minutes. Pour off, rinse under cold water and drain. Cut the thick veins of the leaves flat.
Cut the rest of the savoy cabbage into quarters. Cut 1 cabbage quarter into thin strips. Use the rest of the savoy cabbage for other purposes.
Clean the mushrooms and chop them into small pieces. Peel and finely chop the onions. Heat 1 tablespoon of oil in a frying pan. Sauté the mushrooms, savoy cabbage strips and half of the onions in it for about 5 minutes, remove from the pan.
Knead minced meat, mustard, tomato paste, onion and mushroom mixture and egg. Season with salt and pepper. Form 4 dumplings from the minced meat, place each on a cabbage leaf and roll up firmly, folding the ends in.
Wrap with kitchen string.
Peel and chop the garlic. Shake the thyme dry and remove the leaves. Heat 2 tablespoons of oil in a frying pan. Fry the cabbage parcels for about 5 minutes while turning. Add remaining onions and garlic and steam briefly.
Add the tomatoes, stock and thyme leaves, except for something to garnish, and cook covered for about 40 minutes. Remove the roulades from the roaster. Season sauce with salt and pepper. Remove kitchen string from the roulades.
Arrange roulades with the sauce, sprinkle with thyme.