Preheat oven to 200°C/400°F top and bottom heat.
Wash and halve the potatoes. Cut larger potatoes into thirds. They should be bite-size pieces. Place on a baking sheet and mix with oil, vinegar, salt, and pepper. Bake for about 30 min.
For the dressing, grate lemon zest and squeeze out the juice. Finely dice or use a garlic press on garlic. Mix together into a dressing with the remaining ingredients and season to taste with salt and pepper.
Thinly slice onion and radishes.
Mince fresh herbs.
Take potatoes out of the oven when they are golden brown. This may sometimes take a little longer than 30 min., depending on the type and thickness of the potatoes.
You can serve the salad warm or cold. For a cold salad, let the potatoes cool completely and then mix with the rest of the ingredients in a bowl. For a warm salad, let the potatoes cool a bit anyway (should not be oven hot) and then mix as well.