In small microwavable bowl, mix yogurt, mayonnaise and mustard. Microwave on High 20 to 40 seconds or until warm. Stir; set aside.
In 10-inch skillet, heat 1 1/2 to 2 inches water to boiling. Reduce heat to medium-low. Break each egg into shallow dish; carefully slide egg into hot water. Quickly spoon hot water over each egg until film forms over yolk. Simmer 3 to 5 minutes or until eggs are desired doneness.
Meanwhile, spread about 2 tablespoons sauce on each English muffin half. Top each with half of the spinach leaves.
With slotted spoon, remove eggs from water; place over spinach. Top each with half of remaining sauce; sprinkle with pepper.