Step 1Heat the bacon in a large saucepan over medium heat and cook, stirring occasionally, until crisp, about 12 minutes. Remove the bacon with a slotted spoon and reserve for garnish, leaving the bacon fat in the pot.
Step 2Melt the butter in the bacon fat, then add the thyme, garlic, celery, onion and bay leaf and cook, stirring occasionally, until the aromatics soften, about 6 minutes. Stir in the squash, then cover the pot partially, and cook, stirring occasionally, until the squash is softened and reduced in volume by one-third, about 15 minutes. Uncover, increase the heat to medium-high, and continue cooking, stirring as needed, until the liquid in the pot is concentrated and mostly evaporated, about 10 more minutes.
Step 3Pour in the milk, and bring the chowder to a simmer. Reduce the heat to maintain a gentle simmer, and cook, stirring occasionally, to marry flavors and reduce the milk slightly, about 10 minutes. Remove the bay leaf and then use an immersion or standing blender to puree the soup. Stir in the vinegar and season the soup with lots of salt and pepper.
Step 4Just before serving, thinly shave the patty pan squash crosswise. Pour the soup into bowls and garnish each with a few slices of shaved patty pan, some of the reserved bacon and a sprinkling of parsley to serve.