Wash potatoes well, dab dry, halve lengthwise. Heat oil in a pan, fry potatoes at medium heat for about 10 minutes, turning several times. Divide garlic into the individual cloves, leave the skin on.
Wash the prawns and drain well. Wash and clean the spring onions, cut 1 spring onion into fine rings, the rest into wide rings. Wash the thyme, dab dry, put something aside for garnishing.
Wipe off the rest of the leaves. Add the garlic to the potatoes and fry for another 10 minutes over a gentle heat, turning. Stir together the sour cream, mayonnaise and ketchup. Stir in fine spring onion rings, except for something to garnish.
Season to taste with salt and pepper. Add the prawns, spring onions and thyme and continue to fry for 3-4 minutes while turning. Season to taste with salt and pepper. Sprinkle sauce with onion rings set aside.
Garnish the pan with thyme. Squeeze the garlic out of its skin while eating.