Pat the chicken breasts with paper towels to day and then sprinkle with salt and pepper.
Melt the butter in a saucepan over medium heat. Add the garlic and cook for 1 minute. Add the flour to the butter and garlic and continue to cook for 2 minutes longer, stirring constantly.
Add the chicken stock; cook, stirring, until thickened. Stir in the thyme and bay leaf.
Put the carrot and celery pieces and onions in the slow cooker; arrange the chicken breasts on the vegetables.
Pour the sauce mixture over all.
Cover and cook on low for 5 to 6 hours, or on high for about 2 1/2 to 3 hours.
Add the peas to the pot and replace the lid. Continue cooking on high for 10 minutes.
With a slotted spoon, transfer the chicken and vegetables to a serving dish and keep warm.
Strain the sauce into a medium saucepan.
Bring the sauce to a simmer and continue to cook for 2 to 5 minutes, or until slightly reduced and the flavors are concentrated.
Add the cream. Taste and season with more salt and pepper, as needed.