Remove the backbone from the hen with poultry shears or a sharp knife, place the hen, skin side up, on a work surface, and press down the breastbone to flatten the hen.
In an ovenproof skillet roast the hen, skin side up, in a preheated 450°F oven for 10 minutes.
While the hen is roasting, in a small bowl whisk together the oil, the mustard, rosemary, the garlic, and salt and pepper to taste.
Remove the hen from the oven, spread the mustard mixture over the skin, and roast the hen for 15 minutes.
Transfer the hen to a preheated broiler and broil it about 4 inches from the heat for 2-3 minutes. Now put the mustard seed in with the mustard mixture and broil for 2-3 more minutes(this is so the mustard seeds won't burn).
Transfer the hen to a platter and keep it warm.
To the skillet add the wine and boil the mixture over high heat until it is reduced by half. Add the broth, boil the mixture for 1 minute, scraping up and brown bits, and add any juices that have accumulated on the platter. Strain the sauce through a fine sieve onto 2 plates. Cut the hen in half with a sharp knife, and arrange each half on top of the sauce.