Prepare the patties: In a large bowl, combine the ground beef, pork, sautéed onion, panko, egg, milk, salt, pepper, and nutmeg. Mix gently with your hands until just combined—overmixing makes tough patties. Divide into 4 equal portions and shape into oval patties about ¾-inch thick (thinner edges help even cooking). Make a small indent in the center of each to prevent puffing. Chill in the fridge for 15 minutes to firm up.

Cook the patties: Heat a large skillet over medium-high heat with 1 Tbsp oil. Sear the patties for 2-3 minutes per side until browned. Reduce heat to low, add the red wine and bay leaf, cover, and steam for 8-10 minutes until cooked through (internal temp 160°F/71°C). Remove patties to a plate; reserve the wine liquid (about ½ cup) for the sauce.

Make the sauce: In the same skillet, melt the butter over medium heat. Whisk in flour to form a roux and cook for 1 minute until lightly golden. Gradually whisk in the reserved wine liquid, ketchup, tonkatsu sauce, soy sauce, and beef bouillon. Simmer for 3-5 minutes, stirring until thickened (it should coat the back of a spoon). Taste and adjust saltiness. Add back the patties to warm through for 1 minute.

Assemble the donburi: Divide hot steamed rice into 4 bowls. Top each with a patty, spoon generous sauce over everything (let it soak into the rice—that's the magic). Add optional toppings like a fried egg for extra richness or veggies for balance.