Toss yellow squash, red pepper, asparagus, onion (and any other vegetables you'd like to add) in olive oil. Top with sea salt and fresh ground pepper, to taste.
Spread on roasting pan; roast at 400 degrees for 15 minutes.
Divide spring mix between two large plates.
Top each with half of roasted vegetables and 2 oz. feta cheese.
Drizzle with balsamic vinaigrette.