Add carrots and onions to crock pot (4 or 6 qt) Combine butter, lemon thyme, zest and lemon juice in a small bowl until thoroughly combined. Pour over carrots, season with salt and pepper.
Cook on high for 3 hours, stirring every 45 minutes. Near end of cook time stir in 1-2 tsp of sugar depending on tartness of lemon juice (if desired). Garnish with additional lemon thyme (if desired) and serve.