- Arrange a rack in center of oven and preheat to 300ºF.
- Wrap ham in 2-3 large sheets of foil and transfer to a roasting pan or a wire rack set inside a large rimmed baking sheet. Bake ham until an instant-read thermometer inserted into thickest part registers 115ºF, 3-3½ hours for boneless, 3½-4 hours for bone-in.
- Meanwhile, line a second large rimmed baking sheet with a layer of foil. Arrange pineapple rings on baking sheet in a single layer and set aside.
- Combine red onion and rice vinegar in a medium bowl and toss to combine. Let sit for at least 1 hour.
- Remove ham from oven and increase oven temperature to 425ºF. Unwrap ham, using the foil to line a baking sheet for easy cleanup. Transfer ham to baking sheet. Use a pastry brush to baste ham with Teriyaki Takumi and return to oven for another 15-20 minutes, brushing with more glaze every 5 minutes, until glaze is deeply browned and thermometer registers 125ºF. Carefully transfer ham to a cutting board and let rest while you roast the pineapple.
- Transfer pineapple to oven and roast until tender and starting to lightly brown, 20-25 minutes. Remove pineapple from oven and preheat broiler (do not move oven rack too close to broiler to prevent pineapple from burning). Brush pineapple with a thin layer of Takumi and broil until lightly caramelized, about 2 minutes, watching closely.
- Arrange pineapple and ham on a serving platter and garnish with maraschino cherries if using. Serve with drained pickled onions and buns for building sandwiches.
Recipe and food styling by Christina Chaey.