Heat a large heavy saucepan or Dutch oven over medium high heat. Add bacon pieces and cook, stirring occasionally, until crisp. Remove bacon with a slotted spoon and transfer to a paper towel-lined plate.

Remove all but 2 tablespoons of bacon fat (you can save it for another use) and return saucepan to medium heat.

Add onions and sauté 5 minutes until soft.

Add dried split peas and cook for 1 to 2 minutes until coated with fat. Add chicken stock, 1 cup of water, thyme, and bay leaf and bring to a simmer. Continue to cook, partially covered, for 50 to 60 minutes or until peas are very soft and falling apart.

Optionally, blend all or half of the soup (use caution when blending hot liquids) and return to saucepan. Add a bit of additional water or broth for a thinner consistency if desired. Season with salt and pepper and stir in vinegar to taste. Serve warm, topped with bacon bits and a sprinkle of fresh thyme (if desired).