Japanese Croquettes (Mom‘s Korokke)

Bahan-bahan (16 porsi)

• 2 lb russet potatoes ((about 4 potatoes))
• 1 onion ((9.6 oz, 272 g))
• 1 Tbsp neutral oil
• 1 lb ground beef ((85% lean))
• ½ tsp Diamond Crystal kosher salt ((for the meat))
• ¼ tsp freshly ground black pepper ((for the meat))
• 1 Tbsp unsalted butter ((for the potatoes))
• ½ tsp Diamond Crystal kosher salt ((for the potatoes))
• ¼ tsp freshly ground black pepper ((for the potatoes))
• ½ cup all-purpose flour (plain flour)
• 2 cups panko (Japanese breadcrumbs)
• 3 large eggs (50 g w/o shell)
• 4 cups neutral oil ((or enough for 2 inches (5 cm) of oil in the pot))
• tonkatsu sauce