1) Preheat the oven to 400°F.
2) Heat an ovenproof skillet over medium-high heat. (Because frittata is finished in the oven or under a broiler, be sure your skillet is ovenproof.) When the oil is hot, add the mushrooms and onions and sauté until the onions are browned, and the mushrooms have given up most of their liquid.
3) Add the asparagus and sauté for just a few minutes, until it's becoming tender, but is still bright green. Remove from the heat and set aside.
4) Clean the pan, and grease it lightly.
5) Beat together the eggs, Pizza Seasoning, and 1 cup cheese.
6) Heat the greased skillet until hot. Pour in the egg mixture, add the sautéed vegetables and cook for several minutes, stirring from the edges and bottom of the pan.
7) When the mixture starts to thicken, sprinkle with the crumbled feta cheese; the frittata should be starting to set around the edges.
8) Place the pan in the oven, and bake for 12 to 15 minutes, until the eggs are set through, the cheese is lightly browned, and it's puffy. Remove from the oven, let rest for 5 minutes, then cut into wedges to serve. Garnish with chopped scallions, if desired.
Yield: 6 servings.