Place the chicken, bacon, pearl onions, carrots, celery, chicken broth, thyme, salt, and pepper into the slow cooker. Stir to combine.
Cover and cook on low for 6 hours until the chicken is tender and the vegetables are soft.
Stir in the frozen peas.
In a mixing bowl, whisk together the flour, baking powder, rosemary, and salt. Add the milk and melted butter, then stir until a soft dough forms.
Drop spoonfuls of the dumpling dough evenly over the surface of the hot filling.
Cover and cook on high for 1 hour until the dumplings are cooked through and no longer doughy in the center.