How to Make It
Make the garlic sauce: Combine garlic, lemon juice, and 1 teaspoon salt in a small bowl. Let stand, uncovered, 30 minutes. Whisk in oil.
Make the kombu broth: In a large pot, combine 12 cups cold water and kombu. Cover and bring to a boil over high heat. Immediately remove from heat; let stand 15 minutes. Discard kombu. Stir in 2 teaspoons salt. Bring to gentle simmer over moderate heat.
Make the ponzu: Stir together soy sauce, lime juice, mirin, and 2 tablespoons kombu broth.
To serve, pour the remaining kombu broth into a large heated fondue pot. Serve beef and vegetables alongside for dipping. Cook vegetables in kombu broth until crisp-tender, 3 to 4 minutes. Cook beef in kombu broth to desired degree of doneness, 10 to 15 seconds. If broth reduces, refill fondue pot with water and return to a simmer. Serve with rice, garlic sauce, and ponzu.