For the crust, peel the onion. Cut bacon and onion into cubes. Leave bacon in a pan without fat for about 5 minutes until crispy. After 3 minutes add onion. Add softened butter and breadcrumbs and knead.
Season with salt and pepper and chill. Wash the meat, dab dry and season with salt and pepper. Heat 1 tablespoon of oil in a frying pan and fry meat vigorously on all sides. Cook in a preheated oven (electric cooker: 150 °C/ convection oven: 125 °C/ gas: level 1) for approx. 1 hour.
Wash the potatoes and cook in plenty of boiling water for about 20 minutes. Rinse under cold water, drain and peel. Depending on the size, cut in half or quarters. After about 30 minutes, spread the crust on the roast.
Increase temperature (electric oven: 200 °C/ circulating air: 175 °C/ gas: level 4) and bake for the remaining 30 minutes. For the sauce, wash the herbs, shake dry and, except for a little garnish, pluck the leaves from the stems.
Chop herbs. Finely chop the sour cream, mayonnaise, milk and half of the herbs with a hand blender. Season with salt, pepper, sugar and vinegar. Heat 2 tablespoons of oil in a frying pan and fry the potatoes for 10-15 minutes until crispy brown, turning them over.
Season to taste with salt and pepper. Remove the roast from the oven, let it rest for a short time and cut it into slices. Arrange roast, potatoes and sauce on plates and sprinkle with herbs and garnish.