Add the carrots to a 6-quart slow cooker. Add 2 tablespoons butter, honey (or maple syrup), lemon zest, and salt, stirring to combine.
Cover and cook on low for 4-6 hours, or on high for 2-3 , or until the carrots are just tender. Quickly uncover and stir after the first 2 hours.
Uncover, add the remaining 1 tablespoon butter, stir and taste. Add additional salt, honey maple syrup, or butter if desired. Toss with chopped parsley and flaked sea salt, if using, and serve.
