Allow the steaks to come to room temp, pat dry, and season well with salt and pepper.
Preheat the grill, oil the grates, then lower temp to medium-high.
Heat olive oil in a large sauté pan over medium heat.
Saute onions until they begin to soften, then stir in spinach and cook until wilted and soft.
Add bread crumbs, feta cheese, seasonings and sun-dried tomato pesto.
Cook for 1-2 minutes more until mixture is heated through and well incorporated and set aside while you cook the steaks.
Grill steaks for 5-7 minutes per side for medium-rare, this depends on the thickness of your steak, so use a meat thermometer to check for doneness.
Medium-rare steaks should be 130-135 degrees F, 145 degrees F. for medium. I usually pull mine at 130 degrees, since they will go under the broiler for 1-2 minutes.
Allow the steaks to rest for 3-5 minutes while you preheat the oven broiler. Then top the cooked steaks with the spinach mixture.
Broil, watching carefully, until the topping is heated and just starting to turn lightly golden brown on top, about 1-2 minutes.
Serve immediately.