For the sauce: stir together all sauce ingredients in a bowl until combined, then chill, covered, until ready to use.
For the pork: put oven rack in middle position and preheat oven to 350 degrees.
Cut pork crosswise into 1-inch thick slices medallions.
Whisk together flour, salt, and pepper in a shallow bowl.
Whisk together eggs with a pinch of salt in another shallow bowl and put breadcrumbs in a third shallow bowl.
Working with 1 medallion at a time, dredge them in flour, shaking off excess, and dip in egg, letting excess drip off, then coat with crumbs, pressing to help them adhere.
Arrange pork in 1 layer on baking sheet.
Heat 1/4-inch oil in a 12-inch heavy skillet over moderate heat until hot but not smoking, then panfry medallions, turning over once, until golden brown, 6-8 minutes total.
Transfer to a clean baking sheet and roast in oven until thermometer inserted horizontally into center of meat registers 145-150 degrees, 6-7 minutes.
Serve pork with sauce, lemon wedges and watercress.
Ready in 30 min.