Ceviche Tostadas with Cilantro Cream

For the ceviche:

• 1 pound skinless white fish, halibut, tilapia, whitefish, snapper, bass are good options, cut into 1/2-inch cubes
• 1 small white onion, chopped into 1/4-inch pieces
• 1 1/2 cups freshly squeezed lime juice, from about 12 limes
• 1 large jalapeno, stemmed, seeded and finely chopped
• 1 ripe fat tomato, cored, seeded and cut into 1/4-inch pieces
• 1/4 cup loosely packed cilantro leaves, chopped
• 2 tablespoons extra virgin olive oil
• 1 teaspoon sugar
• kosher salt

For the cilantro cream:

• 3 cups loosely packed cilantro leaves
• 1 tablespoon freshly squeezed lime juice
• 1/2 cup mayonnaise
• kosher salt
• freshly ground black pepper

For serving:

• tostadas
• 1 avocado, halved, pitted, peeled, and sliced
• chopped cilantro