Combine the rhubarb, sugar and 3/4 cup water in a medium saucepan and bring to a boil. Reduce the heat to medium and simmer until the rhubarb is completely broken down, about 15 minutes. Strain through a fine-mesh sieve into a separate saucepan, pressing firmly on the rhubarb to squeeze out the syrup; reserve the rhubarb. Set aside 1/2 cup syrup for glazing.
Make the compote: Add 1/4 cup of the rhubarb solids to the remaining syrup in the saucepan (discard the rest of the solids). Add half of the grapes, the onion, 1 tablespoon mustard, the chopped thyme, red pepper flakes and a pinch of salt. Bring to a boil, then reduce the heat to medium low and simmer, crushing the grapes with a wooden spoon, until the grapes break down and the sauce thickens, about 12 minutes. Add the remaining grapes and 1 tablespoon mustard and simmer 6 more minutes. Let cool completely. (The compote can be made up to 3 days ahead; cover and refrigerate.)
Make the ham: Preheat the oven to 350 degrees F. Mix the brown sugar and the remaining 2 tablespoons mustard with the reserved 1/2 cup rhubarb syrup in a bowl; set aside. Remove the skin and all but about 1/4 inch fat from the ham. Score the fat with a small sharp knife in a tight crosshatch pattern, making the cuts about 1/2 inch apart. Put the ham flat-side down on a rack in a roasting pan. Add 1 cup water to the pan and tent the ham with foil. Roast 1 1/2 hours.
Increase the oven temperature to 375 degrees F. Uncover the ham and brush with the rhubarb-mustard syrup. Continue roasting, brushing with more syrup every 15 minutes, until the ham is golden brown and glazed all over, about 1 1/2 more hours. Transfer to a platter; garnish with thyme sprigs. Serve with the grape-rhubarb compote.
Photograph by Con Poulos
Courtesy of Food Network Magazine