In a small microwavable bol, combine yogurt, maynnaise, and mustard. Microwave on high 20 to 40 seconds or until warm. Stir and set aside.
For poached eggs, in a 10 inch non-stick frying pan, heat 1 2/2 to 2 inches of water to boiling. Reduce heat to medium-low. Break each egg into a shallow dish and then carefully slide egg into hot water. Quickly spoon over hot water onto each egg until a film starts to form over each egg yolk. Simmer 3 to 5 minutes or until eggs have reached desired doneness.
While eggs are cooking spread each muffin half with 2 Tbsp of the prepared sauce, then with half the spinach leaves.
Remove eggs from water with slotted spoon and place each on top of spinach Top each half with remaining sauce and season with salt and pepper.