Soak the roll in water, squeeze it out. Fry the onion in a pan until translucent, mix with the minced meat, egg, mustard, parsley, thyme and rolls and season with salt and pepper. Put the meat dough in the refrigerator for 15 minutes.
Beat the butter until frothy. Finely chop the garlic, add to the butter with lemon juice and herbs and stir in. Season to taste with salt and put in the refrigerator.
Preheat the oven to 180 ° C top / bottom heat (convection 160 ° C).
Take the meat dough out of the refrigerator and shape meatballs out of it with damp hands.
Put the oil in a pan, heat it and brown the meatballs on each side for 1 - 2 minutes. Place the meatballs on a steel plate or in a baking dish. Tear open at the top and fill with herb butter. Place in the oven and bake for about 20 minutes.