Pat chicken breasts completely dry with paper towels. Slice thick breasts in half lengthwise if more than 1 inch thick. Season both sides generously with salt and pepper.
In a shallow dish, whisk together flour, garlic powder, onion powder, 1 tsp salt, and 1/2 tsp pepper. Coat each piece of chicken evenly in the flour mixture, shaking off excess.
Heat olive oil in a large cast iron skillet over medium-high heat until shimmering. Sear chicken in a single layer for 3-4 minutes per side until deep golden brown. Transfer to the slow cooker.
Reduce heat to medium and melt butter in the same skillet. Add shallot and cook 2-3 minutes until softened. Add garlic and cook 30-60 seconds until fragrant.
Add sliced mushrooms in a single layer and cook 4-5 minutes, stirring occasionally, until they release their liquid and brown.
Pour Marsala wine into the skillet and deglaze, scraping up all browned bits from the bottom. Simmer for 2-3 minutes to let the alcohol cook off slightly.
Pour the mushroom-wine mixture over the chicken in the slow cooker. Add chicken broth, thyme sprigs, and bay leaf. Gently stir to combine.
Cover and cook on LOW for 4-5 hours or HIGH for 2-3 hours, until chicken reaches 165°F (74°C) internally and is tender.
Remove chicken to a plate. Discard thyme sprigs and bay leaf. In a small bowl, whisk water and cornstarch until smooth. Stir slurry into the slow cooker liquid.
Pour in heavy cream and stir to combine. Cook on HIGH for 10-15 minutes until the sauce thickens to a velvety consistency.
Return chicken to the slow cooker, nestling it into the sauce. Warm for 5 minutes. Taste and adjust seasoning. Serve over egg noodles, mashed potatoes, or polenta, garnished with fresh parsley.