For the ceviche, rinse the tuna, dab dry and dice finely. Clean, wash and finely dice the celery. Squeeze limes (should yield 10 tbsp. juice). Mix tuna, celery, 3 tbsp. oil and 6 tbsp. lime juice.
Season with 1 teaspoon sugar, cold and pepper. Cover and chill for about 30 minutes.
Wash the tomatoes and dab them dry. Coarsely grate whole tomatoes with skin on a household grater. Mix the tomato pulp with 2 tablespoons lime juice, 5 tablespoons olive oil and 2 teaspoons sugar. Peel garlic and chop very finely.
Stir half the garlic into the tomatoes. Season tomato salsa with salt and pepper. Pour tuna ceviche into 4 glasses. Spread tomato salad over them and cover and chill for about 30 minutes.
For the guacamole, clean the chillies, carve them lengthwise, remove the seeds, wash them and cut them roughly. Halve the avocado and remove the stone. Remove the flesh from the skin. Puree with crème fraîche, rest of garlic, chili and 2 tablespoons lime juice.
Clean and wash the spring onion and cut into fine rings. Wash coriander, shake dry and chop finely. Stir both into the guacamole. Season to taste with salt and pepper. Serve the ceviche with nachos and guacamole.