For the Ceviche:

• 1/2 a red onion, thinly sliced, with the grain
• 1– 1 1/2 teaspoon kosher salt
• 1/4 teaspoon black pepper
• 3/4 cup fresh lime juice (4-6 limes) freshly squeezed
• 1–2 garlic cloves, very finely minced
• 1 fresh serrano or jalapeño chili pepper, seeded and very finely chopped
• 1 pound fresh fish (sea bass, red snapper, corvina, halibut, dorado, escolar, mahi-mahi, tilapia, or hamachi), diced into 1/2 inch cubes
• 1/4–1/2 cup fresh cilantro, chopped
• 1 cup grape or cherry tomatoes, sliced or cut in half
• 1 cup diced cucumber
• 1 tablespoon olive oil (optional)

For the Optional Avocado Sauce:

• 2/3 cup Avocado
• 1/3 cup cilantro
• 2/3 cup water (plus more as needed)
• 1 tablespoon olive oil
• 1/2 teaspoon kosher salt
• 1 teaspoon coriander
• 2 tablespoons lime juice
• 1 garlic clove
• Cracked pepper to taste