Combine garlic, 3/4 teaspoon fresh rosemary, 3 tablespoons olive oil, salt and pepper in large Ziploc bag. Place steaks in the bag, seal the bag and coat steaks with the mixture. Its ok to mush it around a bit to make sure both sides of the steaks are covered. Put in fridge for 45mins (or as long as overnight).
Put 4 tablespoons balsamic vinegar in a very small pan, add the butter and on LOW begin to reduce. swirl pan every few minutes you do NOT want this to burn it will be bitter. this can take about 25mins. VERY LOW HEAT -- it will start to thicken into a glaze.
Take steaks out of the fridge and bring to room temperature.
in a small sauté pan, combine roasted red peppers, 2 tablespoons olive oil, 1/2 teaspoon rosemary, shallots, sherry and red wine vinegar and feta - heat on low/med for 5mins.
Preheat a large sauté pan on medium-high for 2-3 minutes.
Place steaks in the pan (discard bag and marinade) cook 4-5mins and flip. Cover steak with relish and cook the steaks for another 4-5mins. Remove steaks and let rest for about 10mins. Slice steaks and drizzle with balsamic glaze.