Soak the oats for 30 minutes in enough water and drain.
Cut an "X" in the base of the tomatoes. Blanch them for 1 minute in boiling water, transfer them to an ice bath and remove the skin. Cut them in half, remove the seeds and chop the pulp.
Place the tomatoes, oats, cucumber, onion, garlic and pepper in a blender. Blend until smooth (in batches if necessary).
Add the vinegar and mix. Drizzle in the oil while blending to emulsify.
Season with salt, pepper and vinegar to taste. Refrigerate for at least 1 hour before serving.
Serve cold, with garnishes to taste.