Melt the butter in a skillet over medium-low heat. Add the red onions and cook slowly for 10-15 minutes, stirring occasionally, until soft and deep golden brown. Set aside and let them cool slightly. Grill the tortilla on a grill pan over medium heat for 1 minute, just until it has grill marks on the outside. (If you don't have a grill pan, just use a skillet.) The tortilla should still be soft and pliable; just with a little bit of color! Let the tortilla cool slightly.To assemble the wrap, spread the hummus down the middle third of the tortilla. Arrange the cooked onion slices and the sliced roasted red peppers all over the hummus. Add the artichoke heart halves. Toss the salad greens in the balsamic dressing, then lay them on the wrap. Finally, sprinkle the crumbled feta all over the greens. Carefully wrap up the tortilla, tucking in the ends. Slice the wrap down the middle, share with a friend, and chow down!