Meanwhile, in a small jar with a tight fitting lid, shake together the dressing ingredients until well combined. Set aside.
Preheat an outdoor grill or an indoor grill pan over medium-high heat. Brush the onion slices and steaks with olive oil and season with salt, pepper and garlic powder.
Place the steaks and onions on the grill. Grill the steaks until your desired doneness; about 4-5 minutes per side for medium (130-140℉). When steaks are done, move to a plate, cover with foil and let rest for 5-10 minutes.
Grill onions until they are slightly caramelized on the outside, but still have some firmness to them; about 10 minutes.
While the steaks are resting, build your salads. Toss the mixed greens and arugula together on a large platter. Scatter the vegetables and feta cheese on top. Slice the steaks into thin slices, cutting against the grain, and fan them out on top of the salad. When ready to serve, dress with the balsamic vinaigrette.
Notes
Store leftover, undressed salad in an airtight container in the fridge for up to 3 days. Dress leftovers right before serving. Salad that has already been dressed with the balsamic should be tossed, as the lettuce will get very wilted if it sits in the dressing.I like to let each person dress their portion of salad, vs dressing the whole platter. This way everyone can dress it to their liking and any leftovers can be stored for later.