How to Make It
Sprinkle steaks with salt and pepper; let stand at room temperature 20 minutes. Meanwhile, preheat oven to 350°F. Stir together mirin, miso, honey, and chili-garlic sauce in a medium bowl; set aside.
Add oil to a large Dutch oven; heat over medium-high. Add steaks; cook until browned on both sides, about 3 minutes per side. Transfer steaks to a plate; reserve drippings in Dutch oven.
Add fennel, leek, and garlic to Dutch oven; cook over medium, stirring often, until tender, about 5 minutes. Stir in mirin-miso mixture; cook until fragrant, about 30 seconds. Stir in watermelon cubes; top with browned steaks.
Cover Dutch oven; place in preheated oven, and cook until meat is fork tender and pulls away from bone, 1 1/2 to 2 hours. Uncover and cook an additional 30 minutes. Remove from oven; let stand 5 minutes.
Transfer steaks to a serving platter. Spoon watermelon and sauce from Dutch oven over steaks. Serve immediately.