Japanese Stewed Hamburger Steak (Nikomi Hambagu)

Bahan-bahan (4 porsi)

• ½ onion ((5 oz, 142 g))
• 1 Tbsp unsalted butter ((for cooking the onion))
• ¾ lb ground beef and pork combination
• ½ tsp Diamond Crystal kosher salt
• ⅛ tsp freshly ground black pepper
• ½ tsp nutmeg
• ⅓ cup panko (Japanese breadcrumbs)
• 1 large egg (50 g w/o shell)
• 2 Tbsp milk
• 1 Tbsp neutral oil
• ½ onion ((5 oz, 142 g))
• 1 package shimeji (brown beech) mushrooms ((3.5 oz, 100 g))
• 1 Tbsp neutral oil ((for cooking the onion))
• 1 Tbsp unsalted butter ((for cooking the onion))
• 1 Tbsp all-purpose flour (plain flour)
• ½ cup chicken stock/broth
• ¼ cup red wine
• ¼ cup ketchup
• ¼ cup Worcestershire sauce ((I used Lea & Perrins Worcestershire Sauce))
• 2 tsp sugar
• 1 bay leaf
• 4 slices provolone cheese ((optional; I cut a round slice into a square shape for presentation))