Heat the butter and oil in a medium pot over medium-high heat until the butter melts and bubbles. Add the onion and cook, stirring occasionally, until it's translucent and slightly wilted, about 5 minutes. Add the beef and cook, stirring occasionally, just until the pieces are no longer pink on the outside, about 5 minutes.
Add the carrots and potato, stir well, and add 7 cups of water. Bring the water to a boil over high heat, skim off any froth that appears on the surface, then reduce the heat to maintain a strong simmer. Cook, stirring occasionally, until the beef is very tender, about 1 hour. Add the curry paste and salt and continue cooking, stirring and scraping the bottom frequently, for 15 minutes.
Divide the rice in shallow bowls, spoon on the curry and serve immediately.
For curry use a highly traditional boxed product called Vermont Curry. You can find it in Asian markets, International aisle at the grocery store or you can buy it online. The ingredients list is kind of weird, but trust me it's delicious. Servings: 4Adapted from Mastering the Art of Japanese Home Cooking - Morimoto