Preheat oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). Peel, wash and cut sweet potatoes and potatoes into cubes of approx. 2 cm. Season with salt and pepper and mix with 3 tbsp. oil. Spread on a baking tray and bake in a hot oven for about 30 minutes.
Clean, wash and cut the spring onions into rings. Mix vinegar, 4 tablespoons water, 1⁄4 tsp salt and pepper and 1 tablespoon sugar. Fold in 2 tablespoons of oil. Mix vinaigrette with spring onions and hot potatoes and let it soak for about 30 minutes.
Roughly chop the nuts. Clean, wash and shake the lettuce and spinach dry and pluck into bite-sized pieces. Mix potatoes, spinach, lettuce and pecans, then season again with salt and pepper. Crumble the feta and spread on the salad.