Preparation 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. 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). 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. 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. Do Ahead: Croquettes can be formed and coated 2 days ahead. Cover and chill on baking sheet.
Preparation 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. 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). 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. 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. Do Ahead: Croquettes can be formed and coated 2 days ahead. Cover and chill on baking sheet.