In small bowl, stir together yogurt and lemon juice; set aside.
In nonstick skillet over medium-high heat, cook sausage, stirring frequently and crumbling for 2 minutes. Add onions, olives and mushrooms; cook 4 minutes more.
Add spinach leaves and cook until wilted and sausage is fully cooked, about 1 to 3 minutes. Add eggs and cook, stirring constantly until almost dry. Remove from heat and stir in feta. Stuff each pita half with feta-egg mixture. Serve immediately with lemon yogurt.
Line pita with additional spinach leaves, if desired.