Season chicken breasts with salt, pepper, and a sprinkle of Italian herbs.
In a skillet, quickly brown the chicken on both sides (about 2 minutes per side). This step is optional but adds great flavor!
Transfer chicken to the crockpot.
In the same skillet, sauté sliced mushrooms, minced garlic, and chopped shallots until softened.
Add Marsala wine to the skillet, scraping up any browned bits. Let it simmer for 2 minutes.
Pour the mushroom and wine mixture over the chicken in the crockpot.
Add chicken broth and fresh thyme sprigs.
Cover and cook on low for 4-6 hours or on high for 2-3 hours.
30 minutes before serving, mix flour (or cornstarch) with a bit of the sauce to create a slurry. Stir this back into the crockpot to thicken the sauce.
Just before serving, stir in a pat of butter for extra richness and sprinkle with fresh parsley.