1 Squeeze juice from lime and place 2 tablespoons juice in a bowl. Stir in a little Tabasco sauce to taste and the ketchup.
2 Peel onion, chop finely and stir into ketchup marinade.
3 Rinse cilantro, shake dry and pluck leaves. Reserve some leaves for garnish and finely chop the rest. Stir into marinade.
4 Open oysters with an oyster knife. Mix the oysters and their liquor with the sauce and let marinate for about 20 minutes.
5 Halve avocado lengthwise, remove pit and scoop out flesh into another bowl.
6 Add yogurt to avocado and season with salt, pepper and 2-3 teaspoons lime juice, then puree until smooth with an immersion blender. Distribute puree among serving glasses.
7 Rinse tomatoes, wipe dry and cut out stems. Cut tomatoes into quarters, remove seeds and chop tomatoes finely.
8 Sprinkle diced tomatoes over avocado creams. Remove oysters from marinade and arrange in glasses. Garnish with cilantro and serve immediately.