Smoked Trout Croquettes

Name: Smoked Trout Croquettes
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)