Rockfish ceviche with avocado crema

Ingredients

• 455 grams (1 lb) rockfish
• 1/2 red onion, finely diced
• 1/2 cup lime juice
• 1/4 cup lemon juice
• 2 red chilis, finely diced
• 1/2 fennel bulb, finely diced
• 1/4 cucumber, finely diced
• 1/2 yellow pepper, finely diced
• 1 pint cherry tomatoes, halved
• 1/4 bunch cilantro, finely chopped
• 1/4 bunch parsley, finely chopped
• Pinch of salt
• 1 avocado
• 1/2 cup sour cream
• 6- 8 tostadas
• Lime, for ganish
• Olive oil, for garnish
• 1 orange, zested