Preparation

For the crepes, in a medium mixing bowl whisk together flour, ¾ cup milk, eggs, melted butter substitute, and salt until smooth. Cover and refrigerate 30 minutes.   Preheat a 10-inch nonstick skillet over medium-high; lightly coat the pan with nonstick spray. Stir the crepe batter and pour about 2 tablespoons into the hot pan, lifting the pan off the heat, tilting and rotating it so that the batter forms a thin, even layer. Cook until the top of the crepe is set and the underside is golden, about 45 seconds. Turn the crepe over using a spatula or your fingers (fingers work best here) and cook until the second side is lightly browned, about 5 seconds. Transfer the crepe to a piece of wax paper. Cook the rest of the crepes in the same manner, coating the pan with spray and stirring the batter before starting each one. Stack the finished crepes between sheets of wax paper. (Crepes may be made up to 1 day in advance; store in a resealable plastic bag in the refrigerator.)   For the filling, whisk together eggs and milk in a small bowl; set aside. Heat olive oil in a 10-inch nonstick skillet over medium. Add mushrooms and onion and cook, stirring often, until mushrooms are softened and onion is translucent, about 10 minutes. Stir in spinach and egg mixture and cook, stirring constantly, until egg is set. Off heat, sprinkle eggs with cheese.   To serve, arrange a crepe on each of 4 serving plates. Divide egg mixture among the crepes, then fold each crepe into a triangle; serve immediately.