If your slow cooker has a saute function, turn it on. If not, heat a large skillet over medium-high heat.
Add the butter to the slow cooker/skillet and brown the chicken breasts 3-5 minutes on each side. They just need to be browned, not cooked through.
Add the shallot, rosemary, and garlic to the slow cooker/skillet and cook another 1-2 minutes.
If using skillet, transfer chicken and rosemary/garlic mixture to the slow cooker.
Pour in the chicken broth and stir in the lemon zest. Season with salt and pepper.
Cook on high for 3-4 hours or low for 6 hours, until the chicken is tender.
Remove the chicken from the slow cooker and set aside on a plate, covered to keep warm.
Again, if your slow cooker has a saute function, turn it on. If not, carefully transfer the juices from the slow cooker to a large skillet over medium heat.
In a small bowl, whisk together cornstarch and 2 tablespoon of water.
Add the cream to the slow cooker or skillet, and then stir in the cream and cornstarch slurry.
Cook for ~5 minutes, until the sauce is thickened.
Drizzle the sauce over the chicken. Garnish with additional rosemary, if desired. Enjoy!