Balsamic reduction: 

Bring the vinegar to a boil, then reduce the heat to very low and simmer gently for approximately 20 minutes. The vinegar should reduce to ⅓ cup and will thicken and become syrupy. Set the reduction aside.

Soup:

Place the pears, stock, salt and pepper in a pot and bring to a boil over high heat. 

Reduce heat and simmer for 12 to 15 minutes, until the pears are fork-tender.

Puree the soup using an immersion blender or in small batches in a blender.

To serve, garnish with the balsamic reduction, crumbled feta and chopped, toasted pecans.