Drain the mozzarella. Quarter melon lengthwise and remove seeds. Cut the flesh first from the skin, then into bite-sized pieces. Wash the basil, shake dry and chop the leaves finely.
Mix mozzarella and about 18 melon pieces with basil, oil, 1 tbsp. lemon juice, some salt and pepper. Leave to stand for about 10 minutes.
Meanwhile peel and chop the onion and garlic. Blend the tomatoes together with the juice, tomato paste, remaining melon and 2 tbsp. lemon juice in a blender. Season to taste with salt, pepper and a little sugar.
Keep cool until serving.
Put the marinated melon pieces and mozzarella alternating on wooden skewers. Serve with soup in glasses.