Step 1

Preheat oven to 325 degrees, with a rack in lower third. Prepare an ice-water bath. Bring a pot of water to a boil. Trim 1/4 inch off root end of each onion, then cook onions in pot until just beginning to soften, 3 to 4 minutes. Transfer to ice-water bath, slip off skins, and cut into quarters. (Quartered onions can be refrigerated up to 2 days.)

Step 2

Stir together molasses, brown sugar, honey, mustard, allspice, 1/4 teaspoon salt, and 1/8 teaspoon pepper. Place ham in a large roasting pan, cut-side down; baste generously with glaze. Scatter onions around ham and pour in 1/2 cup water.

Step 3

Cover ham with parchment-lined foil; roast until a thermometer inserted into thickest part (not touching bone) reads 125 degrees, 1 hour to 1 hour, 15 minutes. Uncover, baste with more glaze, and return to oven. Increase heat to 400 degrees and roast, basting and rotating pan once, until outside is crisp, 15 to 20 minutes more. Transfer to a serving platter; tent with foil.

Step 4

Return roasting pan with onions to oven; roast until sauce has thickened slightly and onions are caramelized, 15 to 20 minutes more. Transfer onions and pan drippings to a serving vessel; skim fat from top. Serve ham with biscuits, mustard, onions, apples, and parsley.