1. If desired, stack spinach leaves and cut into 1/2-inch-wide strips. Mound spinach equally onto plates.
2. In a 10- to 12-inch nonstick frying pan over high heat, stir turkey bacon until browned and crisp, about 5 minutes. Spoon equal portions of the bacon over spinach.
3. To pan, add onion, corn, vinegar, and sugar. Stir mixture until onion is limp, 3 to 4 minutes. With a slotted spoon, transfer onion and corn onto spinach, then spoon cooking liquid over salads.
4. Arrange blue cheese equally on salads and add croutons.