Mazatlán Ceviche de Sierra (Sinaloan Mackerel Ceviche)

Using a very sharp knife, cut the fish into 1/2-inch cubes. Transfer to a large plate or baking sheet in a single layer and freeze until the fish is firm to the touch but not fully frozen, about 30 minutes. Meanwhile, place a food processor bowl and its blade into the freezer to chill while the fish is freezing. Serious Eats / Lorena Masso Toss carrot with 1/4 teaspoon salt and transfer to a fine-mesh strainer set over a bowl. Let drain for 20 minutes, gently pressing on the carrots to remove excess moisture. Serious Eats / Lorena Masso Once the fish is firm, transfer to the food processor and pulse until evenly ground, about 10 to 20 pulses (fish should be roughly 1/8 inch in size; be careful not to over-process fish to a smooth paste). Serious Eats / Lorena Masso Transfer fish to a mixing bowl and toss well with onion, lime juice, remaining 1 1/2 teaspoons salt, and pepper to taste. Let marinate in refrigerator, stirring occasionally, until the fish turns from translucent to opaque white, 5 to 10 minutes. Serious Eats / Lorena Masso Stir in the drained grated carrot and cilantro. Season with salt and pepper to taste. Top with cucumber and serve right away with tostadas, mayonnaise, avocado, and sauces. Serious Eats / Lorena Masso