Peel and press the garlic. Peel and wash the potatoes and cut them into cubes of about 2 cm. Onions peel and cut into approx. 5 mm thick slices. Leave slices whole. Mix potatoes and garlic with oil in a bowl.
Season with salt and pepper. Spread on a baking tray lined with baking paper. Place the onion slices on top and cook in the preheated oven (electric cooker: 250 °C/ convection oven: 225 °C/ gas: see manufacturer) for about 25 minutes.
In the meantime, wash lemon hot, grate dry and finely grate the zest of 1/2 lemon. Halve the lemon and squeeze 1 tbsp. juice. Use the rest of the lemon for other purposes. Wash parsley, shake dry and chop finely.
Mix the crème fraîche with milk, sugar, parsley, lemon juice and grated lemon. Season to taste with salt and pepper. Clean, wash and spin dry the salad.
Wash and drain the tomatoes. Add to the potatoes about 10 minutes before the end of the cooking time. Take the tray out of the oven and let it cool lukewarm. Remove the garlic from the tray, add to the dressing and puree finely.
Mix potato mixture and salad and arrange in a bowl, drizzle with dressing.