Preheat grill pan to high heat. Remove meat from the refrigerator and unwrap. Let beef stand a few minutes, to take the chill off.
Heat a medium skillet over moderate heat. Add butter and sliced onions to a hot pan and season with salt. Saute onions for 25 minutes, stirring occasionally, until onions are caramel in color, tender, and sweet.
Meanwhile, place peppercorns in a heavy-duty food storage bag. Hit peppercorns with a small, heavy frying pan to crack them. Baste meat with a little Worcestershire sauce on each side using a pastry brush. Coat steaks on 1 side with cracked black peppercorns. Grill steaks 5 minutes on each side for medium rare.
Meanwhile, heat leftover mashed potatoes on high in microwave oven for 5 minutes. If you are using instant potatoes, prepare to package directions and transfer warm potatoes to a mixing bowl. Stir cheese and scallions into the mashed potatoes. Heat a nonstick skillet over moderate heat. Coat the inside of pan with butter to aid in crusting. Spread cheesy potatoes in an even layer in the skillet and allow them to crust and brown on the bottom, 5 to 7 minutes. Cut the potato layer in half gently with a butter knife, careful not to scratch your nonstick surface. Fold one half over top of the opposite making a half moon shaped potato pie with a crusty outside and a cheesy, soft center.
While steaks and potatoes cook, brown chopped bacon in the small frying pan you used to crack the peppercorns over high heat. Transfer browned bacon bits to a paper towel lined plate to drain. Wipe out skillet and return to heat. Add 1 /2 cup balsamic vinegar to the pan and bring to a bubble. Reduce heat to low and simmer 1 to 2 minutes, until vinegar is reduced by half and thick. Turn off heat. Pour spinach into salad bowl. Top with bacon and mushrooms. Whisk extra-virgin olive oil into warm vinegar and pour over salad. Toss salad and serve.
Season the cooked steaks with a little salt and remove to a carving board. Let the steaks rest for 5 minutes for the juices to redistribute through the meat evenly. If you slice into a hot steak, the juices will run out and the first bite will be the only tasty one.
Slice steaks thin against the grain of the meat. Smother steak slices in onions. Cut potato pie into 4 wedges and serve along side steaks with spinach salad.