Mix mustard, soy sauce, garlic, rosemary, thyme and ginger together in a bowl. In a steady, slow stream, whisk in olive oil until well-combined and emulsified, like a mayonnaise-like cream.
Paint the lamb all over with the mustard mixture and set it on rack, fat side up, in roasting pan. Let sit at room temperature for 2 hours before roasting.
Preheat oven to 425º. Scatter chopped onion, lemon slices and fresh herbs in bottom of pan, under the rack. Roast for 30 minutes and then turn oven temperature down to 350º. Roast until desired doneness, 135º for medium-rare, 145º for medium; about 1½ hours more. Add about 2 cups chicken stock to the pan after 1 hour and add some more as needed after about another ½ hour. Pull the roast out of oven 5 degrees shy of goal temperature, transfer to cutting board or platter, tent with foil and let rest 20 to 30 minutes.
Meanwhile, make the gravy. Strain and de-fat pan juices and drippings into large measuring cup and set aside. If necessary, add more chicken stock to make at least 2 cups. Make a slurry by mixing flour and water until a thick liquid, but not a paste. Place roasting pan on stove and turn 1 to 2 burners on high. Pour wine into pan and deglaze, scraping up any browned bits, and reduce by half. Pour reserved pan juices into pan and whisk to combine. Slowly pour slurry into pan while whisking constantly until pan sauce starts to thicken; cook at a steady simmer until thickened and flour taste is cooked off, about 5 to 10 minutes. Taste for salt and pepper.
Snip netting/string on roast and carefully lift out roast, removing the string. Slice lamb and serve with gravy on the side.