Clean and wash the chard, remove the base of the stem. Blanch the leaves, except for the small inner leaves, briefly in boiling salted water, rinse with cold water and spread out on kitchen paper to drain.
Peel and finely chop the garlic. Separate the egg. Knead veal, egg white, 2 tablespoons cream and garlic, season with salt and pepper. Clean and wash the tomatoes, remove the stalk and cut 5 tomatoes into thin slices.
Lay out half of the chard leaves slightly overlapping on a tea towel to form a square measuring approx. 25x25 cm. Spread half of the sausage meat mixture on each side and cover with tomato slivers. Repeat this layering process and roll up the leaves firmly using the cloth.
Carefully place the roll with the seam facing down in a large pot of boiling salted water. Let it simmer at low heat for 30-35 minutes. Melt the fat and sweat 3 tablespoons of mustard, mustard seeds and flour in it.
Pour in broth and milk and let the sauce simmer for about 10 minutes. Mix the remaining cream and egg yolk, stir into the sauce, do not let it boil anymore. Season the sauce with lemon juice, mustard, salt and pepper to taste.
Seed the remaining tomato and dice finely. Arrange chard roll, some sauce, tomato cubes, remaining chard leaves and lemon slices on a plate. Serve the rest of the sauce with it.