Briefly put the tomatoes in boiling water, rinse, peel and core them. Then cut into small cubes. Cut the spring onions into thin slices. Peel the avocados and remove the pulp from the stone. Dice one avocado, mash the other with a fork. Mix the avocados with the crème fraîche, lime juice, mustard, tomatoes and leeks and season to taste with pepper, salt and chili powder.