Prepare the ingredients: Thinly slice 1 large onion, peel and finely mince 2 carrots (or use a food processor), slice or halve 3.5 ounces mushrooms, and chop 2 cubes Japanese curry roux.
Sear the beef: Heat a large pot over medium heat and add oil. Remove the beef from the marinade (do not discard the marinade). Sear the beef quickly on the surface, then remove it from the pot. The inside does not need to be fully cooked at this stage.
Sauté the onions: Melt 2 tablespoons unsalted butter in the same pot and sauté the onions over medium-high heat for about 5 minutes, or until golden brown.
Cook the vegetables: Add the carrots, mushrooms, and 1 14.5 ounces canned diced tomatoes. Sauté over medium-high heat for about 5 minutes, or until the tomatoes soften.
Add the marinade: Pour in the reserved marinade and bring to a boil.
Simmer the curry: Add the seared beef, 2 cups water, 2 teaspoons bouillon paste, and 1 bay leaf. Bring to a boil, then reduce the heat to low, cover with a lid, and simmer for 2 hours.
Add the curry roux: Once the curry has finished simmering, turn off the heat. Add the chopped curry roux and mix well until it melts.
Final simmer: Turn the heat to low again and simmer for about 10 minutes, or until the sauce thickens.
Serve: Serve over warm, cooked Japanese rice and enjoy!