Ingredients

• 6 medium russet potatoes (about 2 1/2 lb. total), peeled, quartered
• Kosher salt
• 1/4 cup (1/2 stick) unsalted butter, cut into pieces
• 1 oz. Parmesan, finely grated (about 1/4 cup)
• 1/4 cup chopped chives
• 2 Tbsp. prepared horseradish
• 1/2 tsp. freshly ground black pepper
• 4 large eggs, lightly beaten, divided
• 8 oz. smoked trout, skin and any large pin bones removed, shredded
• 2 oz. low-moisture mozzarella, coarsely grated (about 1/2 cup)
• 2 cups panko (Japanese breadcrumbs)
• Canola or vegetable oil (for frying; about 6 cups)
• Lemon wedges (for serving)

Instructions

1. Cover potatoes with cold salted water in a large pot. Bring to a boil over medium, covered, then reduce heat as needed to maintain a simmer until potatoes are tender, 20–30 minutes. Drain and transfer to a large bowl.
2. Mash warm potatoes with butter, then stir in Parmesan, chives, horseradish, pepper, 2 beaten eggs, and 1/2 tsp. salt until smooth. Fold in trout and mozzarella. Form potato mixture into about 60 round croquettes (about 2 Tbsp. each).
3. Place remaining 2 beaten eggs in a shallow bowl. Place panko in another shallow bowl. Dip 1 croquette into egg, letting excess drip off, then roll in panko to coat. Transfer to a parchment-lined rimmed baking sheet. Repeat with remaining croquettes.
4. Pour oil into a large pot to a depth of 4". Heat over medium-high until it sizzles when you toss in a pinch of panko or a deep-fry thermometer registers 350°. Working in batches, fry croquettes, turning occasionally, until golden brown, 4–5 minutes per batch. Transfer to paper towels to drain; season with salt. Serve immediately with lemon wedges alongside.
5. Do Ahead: Croquettes can be formed and coated 2 days ahead. Cover and chill on baking sheet.