Slice red onion and salt generously, and let stand 15 minutes until it begins to release its liquid. ( this will removed some bitterness). Rinse well.
Place shrimps, scallops, garlic, onion, salt, pepper, chili, and lime juice in a shallow bowl, gently mix, and marinate in the refrigerator for at least 20 minutes before serving.
Before serving, add cilantro, cucumber and tomato and a drizzle of olive oil, gently mix. Taste for salt.
To make Avocado Sauce, blend the avocados, 1/3 cup cilantro, water, 1 tablespoon olive oil, salt & pepper, coriander, 1/8 cup lime juice and 2 garlic cloves in a blender until smooth, adding a bit more water if needed to get the blades going.