optional garnishes: halved grape tomatoeschopped chives or parsley
Heat oil in a 10-inch cast iron skillet (or nonstick) over medium heat. Add onion and cook a couple minutes, stirring occasionally. Add garlic and asparagus (if using) and cook until vegetables are tender, 5-8 minutes. Add cooked chicken and spinach, stir, cover and let warm through to wilt while mixing the eggs.
In a 4-cup measure, mix together the eggs, water and seasonings. Stir in 1/4 cup of the crumbled feta.
Pour the eggs evenly over the top of contents of skillet. Do not stir. Cover and let the eggs set around the edges and somewhat in the middle.
Turn on the oven broiler to preheat a bit.
Using a thin spatula, lift the edges and tip the pan so the liquid egg from the middle runs to the edge and under the cooked egg. Continue all the way around the edge until most of the egg is cooked (or use the spatula to break through the center a bit in places to allow the uncooked egg to flow through).
Put the pan under the broiler (covering the handle with foil if it is not metal). Let it broil 3-5 minutes or until all the egg is cooked and the top is browned.
Sprinkle the remaining cheese over the top along with optional garnishes, if using, and serve.