This is really an easy, wonderfully flavored dish to prepare for a large dinner party. Beef is a deeply flavored meat that does not require a bunch of herbs or spices to enhance the dish. Salt and pepper and the proper preparation can always produce spectacular results. This is certainly a lot cheaper to prepare than prime rib and will please your guest equally well. This is a keeper.
Place first 6 ingredients in a rectangular baking pan that can withstand high heat (preferably stainless steel). The sides should not be really high. A lasagna pan works well. Do not use aluminum due to the vinegar. Toss onions to coat well with oil and seasonings. Place in a 400 degree oven to roast for 1 hour. Stir several times.
Remove the steaks from the fridge to remove the chill, as the onions roast. Rub 1 tsp cracked pepper on each side of steak (total 4 tsp). 20 minutes before the onions are cooked, salt each side of steaks with 3/4 tsp kosher salt (3 tsp total).
After onions have roasted for 1 hour, remove from oven and place the two steaks (roasts) on top of the onions. Increase the oven temperature to 475 degrees. When oven has reached the higher temp, place baking pan with the onions and steaks in the oven for 38 -48 minutes. Reduce heat to 400 degrees after the first 25 minutes of roasting. This time frame will produce a medium rare to medium doneness. When in doubt, use a thermometer.
When meat is removed from the oven, remove the meat from the roasting pan and place on a large platter to rest 15 minutes before carving. Slice narrowly against the grain. To serve, spoon some of the onions and juice on each portion.