Place a Dutch oven or large pan with high sides over medium heat. Add 4 tablespoon butter. When butter melts, add onion and saute until tender, 6 to 7 minutes.
Stir in 1 cup flour, 1 teaspoon dried thyme, and garlic. Cook, stirring, until no dry spots remain, about 2 minutes.
Slowly pour in 3 cups stock, whisking as you add the stock. Bring to a simmer, scraping up any dried bits on the bottom of the pan. Simmer until mixture turns slightly golden and gets very thick, 8 to 10 minutes. It should resemble a thick gravy. (See notes below if you want to freeze this meal.)
Transfer contents of the pan to the bowl of a slow cooker.
Add the remaining 3 cups chicken stock, chicken thighs, carrots, celery, and bay leaves.
Cook on low until chicken is tender, 4 to 5 hours. (At this point the stew can hold on the warm setting for up to 2 hours. Note: If your stew was not fully defrosted before it was added to the slow cooker, be sure to check that the chicken has cooked all the way through before serving.)
Remove lid from slow cooker. Taste broth and season with some additional salt, if needed.
Transfer chicken to a cutting board and shred or chop. Return chicken to the slow cooker and add peas. Increase slow cooker heat to high.
Meanwhile, whisk together 1 ¾ cup flour, baking powder, parmesan cheese (if using), ¼ teaspoon dried thyme, and salt. Add milk and melted butter and stir just until everything is combined.
Spray a ¼ cup measuring cup with nonstick cooking spray. Use the measuring cup to scoop and drop the biscuit batter into the stew. (No need to be precise about this. The biscuits will spread out as they cook.)
Cover slow cooker and continue cooking until dumplings are cooked through, about 30 minutes.