Add the onion, garlic, olive oil, vinegar, mustard, lemon juice, and dried spices and salt and pepper to a small bowl and whisk to combine. 

Place the American leg of lamb in an oven safe baking dish and pour the marinade on top. Turn the lamb to coat and let marinate for at least an hour, or up to overnight in the refrigerator.

Preheat your oven to 350-degrees F.

While the oven is preheating, roll the leg of lamb up into a uniform shape and snugly tie it at both ends with two lengths of kitchen twine. Return the roast, seam side down, to the baking dish. 

Roast the lamb, uncovered, for 40 minutes. Remove the lamb from the oven and add the beans to the pan. Stir them to coat in any juices that have accumulated. Drizzle the beans with 1 Tablespoon olive oil and sprinkle with a pinch of salt and pepper. 

Return the lamb and beans to the oven and continue roasting for 10-30 minutes, until an instant-read meat thermometer registers 145 F (for medium-rare) or 150 F (for medium) when inserted in the thickest part of the meat. 

Remove the lamb from the oven, cover with foil, and let stand for 15 minutes before serving.

Meanwhile, make the gremolata. Finely chop the parsley, zest the lemon, and mince the garlic. Combine all three in a small bowl and season with a pinch of kosher salt.

Carve the lamb into slices against the grain. Plate with the beans, top with a spoonful of gremolata and serve.