Wash the potatoes and cook in plenty of boiling water for about 20 minutes. Rinse under cold water, drain and peel while still hot.
Peel and finely chop the onion. Whisk 4 tbsp. oil, stock, vinegar and onion. Season with sugar, salt and pepper. Cut potatoes into slices and toss in the marinade. Clean and wash spring onions and cut them diagonally into rings.
Cut the avocados in half, remove the seeds. Remove the avocado halves from the skin. Sprinkle the flesh with 2 tbsp. lemon juice and cut into slices. Wash and halve the tomatoes.
Carefully mix the avocado, spring onions and tomatoes with the potatoes. Mix mayonnaise, 3 tbsp. lemon juice and milk. Season with salt and pepper. Pour the dressing over the salad. Wash the fish, dab dry and portion.
Season with salt. Heat 3 tablespoons of oil in a frying pan and fry the fish for about 4 minutes on each side. Arrange fish and salad on plates. Add lemon wedges.