Cut each chicken breast into 2-inch chunks, dice the onion, and mince the garlic. Measure broth and seasonings.
Place chicken chunks in the crockpot. Scatter onion and garlic on top. Pour broth over everything, then sprinkle with salt, pepper, and thyme.
Cook on low for 8 hours. At 7 hours, check that chicken is submerged and broth is gently simmering.
In a small bowl, whisk cornstarch with cold water until smooth.
Pour cornstarch slurry into the crockpot and stir well. Add butter (cut into small pieces) and heavy cream; stir until smooth and glossy.
Cook on low for another 30 minutes until gravy thickens. Taste and adjust salt/pepper. Serve over mashed potatoes.