In a large pot or Dutch oven, heat the olive oil over medium heat. Add the onions, carrots, and celery, sautéing until the onions become translucent (about 5 minutes).
Add the garlic and cook for another 30 seconds until fragrant. Immediately add the chicken broth, cooked chicken, bacon (if using), thyme, and bay leaf.
Bring the mixture to a gentle boil, then reduce to a simmer and cook for 15 minutes to allow the flavors to meld.
While the soup simmers, whisk together the flour, baking powder, and salt in a medium bowl.
Using your fingertips or a pastry cutter, work the cold butter into the flour mixture until it resembles coarse crumbs.
Gently stir in the milk and herbs until just combined. Do not overmix.
Using a tablespoon, drop spoonfuls of the dumpling mixture directly into the simmering soup. Aim for 8-10 evenly sized dumplings, leaving space between each.
Cover the pot with a tight-fitting lid and cook for 15 minutes without peeking.
Once the dumplings are cooked through (they should be firm but tender when pierced with a toothpick), remove the bay leaf.
Taste the soup and adjust seasoning with salt and pepper as needed.
Ladle the hot soup and dumplings into bowls, garnish with fresh parsley, and serve immediately.