To brine ham: Heat water, 2 cups brown sugar, 1 cup salt, cloves, allspice and bay leaves in a large pot over medium-low heat, stirring often, until the sugar and salt are dissolved. Add ice and stir to cool the liquid.

Score fat of ham into a 2-inch-wide crosshatch pattern without cutting into the meat. Submerge in the brine. Refrigerate for 2 to 3 days.

To roast ham: Preheat oven to 450 degrees F.

Pound garlic, thyme and salt to a paste in a mortar and pestle (or in a mini food processor). Transfer to a bowl and stir in oil and crushed red pepper. Remove the ham from the brine and thoroughly pat dry with paper towels. Rub the paste all over the ham and into the scored cuts.

Trim the root end and top inch of greens from leeks; cut in half lengthwise. Submerge them in a large bowl of water and gently separate the layers to remove any grit, while keeping each piece together. Drain well.

Arrange a layer of leeks in a large roasting pan. Set the ham on top, with the fat cap facing up. Roast in the middle of the oven for 20 minutes.

To prepare glaze: Bring jelly, honey, ginger and brown sugar to a simmer in a small saucepan, stirring, to melt the sugar. Remove from heat.

After 20 minutes, brush the ham with 1/4 cup of the glaze.

Reduce oven temperature to 300 degrees . Roast, brushing with 1 tablespoon glaze every 30 minutes, until an instant-read thermometer inserted into the thickest part without touching bone registers 150 degrees F, about 3 hours.

Transfer the ham to a clean cutting board, loosely tent with foil and let rest for 20 minutes before slicing. Serve with the leeks, if desired.