Fresh ham does not have the smoky, woody flavors of a cured ham; rather, its flavor is delicate and mingles well with herbs.
Heat oven to 325 degrees with rack positioned as low as possible. To make the green-herb paste, combine garlic, rosemary, oregano, 1 tablespoon salt, and 2 teaspoons freshly ground black pepper in the bowl of a food processor; pulse to combine. With machine running, add olive oil through feed tube, and process until just combined but some texture still remains. Green-herb paste can be made 1 day ahead and stored in an airtight container, refrigerated, until you're ready to cook the ham.
To make the ham, you will need a large roasting pan; we used a 14-by-18-inch one. If you can cook a 20-pound turkey in the pan, the ham will fit; just make sure the pan fits in your oven. Arrange onion rounds in the bottom of the pan. Place 8 rosemary sprigs and 2 dozen bay leaves on top of the onions; this onion-herb bed will keep the ham from burning and will make an aromatic gravy.
While ham is still cold, score the skin and fat: Using a sharp slicing knife, cut large diamonds, spaced about 2 1/2 inches apart, through both the skin and the fat. The uncooked meat should be moist and reddish pink, and the fat should be very white and smooth. Don't be alarmed by the amount of fat under the skin; it melts, shrinks, and crisps while cooking, making the meat tender. Using a paring knife, make a slit in the meat at each intersection; insert a sliver or two of garlic and a tuft of rosemary into each.
Transfer ham, scored-side up, to pan. Using your fingers, rub 1/2 cup green-herb paste into the cut areas, over the skin and over the exposed end piece. Sprinkle liberally with salt and pepper.
Tie ham crosswise in 2 or 3 places with kitchen twine. Make a bay wreath around the shank bone: Tie a piece of twine around the bone; tuck remaining bay leaves under the twine. Let ham sit at room temperature until no longer cold to the touch, about 30 minutes; if it goes into the oven too cold, it won't cook through. Cook ham 1 hour. Slide out the pan on the rack. Slowly pour 3/4 cup wine over ham; rub more paste into cut areas as they expand and release juices. Bake 3 1/2 to 5 hours more, basting with wine every hour and rubbing in more paste as needed, until an instant-read thermometer registers between 145 degrees and 155 degrees when it is inserted into 2 different sections of the center; be careful not to insert the thermometer next to the bone, or the reading will be incorrect. Cooking times will vary with the size of the ham, the weight of the pan, and the heat of the oven, so an instant-read thermometer is essential. While roasting, reverse orientation of pan for even browning, and tent ham with foil if the outside is cooking too quickly. Remove ham from oven, and let rest; the internal temperature will rise 5 degrees to 10 degrees more.
When cool enough to handle, transfer ham to a platter garnished with rosemary and dandelion greens; remove twine. For easiest slicing, let ham rest 1 to 1 1/2 hours. Discard onions and rosemary; pour remaining liquid into a fat separator. Place pan over high heat. Add 1/2 cup water; once liquid bubbles, scrape bottom with a wooden spoon to dislodge any cooked-on bits. Cook until mixture reduces slightly, about 5 minutes. Remove fat from reserved liquid, and add liquid to pan. Heat through, strain, and serve with the ham.
To serve fewer than 12 people, buy just the shank end of a fresh ham; prepare as described above, and check for doneness with an instant-read thermometer after 3 to 4 hours.