1 For the potatoes: Grease baking sheet with butter and preheat oven to 220°C (approximately 425°F). Scrub potatoes well, dry and place on the baking sheet. Brush with olive oil and season with salt and pepper. Bake for about 35 minutes or until golden brown.
2 For the burgers: Soak the bread in warm water. Peel onion, finely dice and saute in 1 tablespoon oil. Squeeze soaked bread, tear into small pieces and mix with the meat. Season with salt and pepper and form into 4 patties with moistened hands. Heat oil in a pan and sear the burger patties on both sides over high heat, then reduce heat and cook for another 2-3 minutes (longer if desired).
3 Rinse, trim and slice the radishes.
4 Split the wheat buns and toast lightly if desired. Place burgers on lower halves, top with creme fraiche, radish and sprouts. Close the rolls. sandwich with top bun halves. Serve with potatoes.