In a covered medium saucepan cook spinach in a small amount of boiling, salted water for 3 to 4 minutes or until tender. (Or place spinach in a microwave-safe casserole with 1 teaspoon water. Cover and microwave on 100 percent power [high] for 1 to 2 minutes or until tender, stirring once.) Drain spinach thoroughly.
In a medium bowl beat eggs well. Add drained spinach; continue beating until thoroughly mixed.
Heat half of the butter in a 7- or 8- inch omelet pan or skillet with flared sides over medium-high heat until a drop of water sizzles. Pour half of the egg mixture into pan. As eggs set, run a spatula around the edge of the skillet, lifting eggs and letting uncooked portion flow underneath. Cook until top of omelet is set, but still shiny. Turn omelet; sprinkle with half of the cheese. Cook 2 minutes.
Transfer omelet to a warm serving plate; roll up the omelet. Cover and keep the omelet warm in a 300 degree F oven while you're repeating step 3 to make a second omelet. Makes 2 servings.
To reduce the fat and cholesterol in this egg dish, substitute 1-1/2 cups frozen or refrigerated egg product, thawed for the 6 eggs. Then, use half as much crumbled feta cheese (1 ounce total).