In large, ovenproof pan, heat a little oil, then sear beef both sides, until nicely browned. Add potatoes, carrots, baby onions, yellow, and red bell peppers and mix well with beef juices. Place pan in oven for 15 minutes.
Remove from oven. Remove beef and vegetables from pan. Stem and seed the peppers. Cut the meat and vegetables into bite size pieces and set aside, keeping warm. Deglaze pan with sake. Add soy sauce and mirin. Thicken, if necessary, with cornstarch and water. Cover meat and vegetables with gravy. Sprinkle chive on top and drizzle chili oil on top, to taste.