Combine Alfredo sauce, mustard and lemon juice in small saucepan; heat over medium-low heat, stirring frequently. Hold hot until service.
For each sandwich, toast 2 slices bread. Spread 1 1/2 tablespoons Alfredo Dijon sauce evenly over 1 slice toast. Top one slice with 1/2 ounce spinach, 1 slice onion, 2 slices tomato and 2 ounces turkey; cover with remaining slice toast.