Melt 3 tablespoons of butter in a large pot or Dutch oven over medium heat. Stir in the cabbage and onion. Cook and stir until the cabbage is tender, about 20 minutes.
Place the bacon in a large, deep skillet, and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain the bacon slices on a paper towel-lined plate. Cut the bacon into bite-sized pieces; set aside.
Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling, stir in the pierogies and return to a boil. Cook until the pierogies float to the top, about 5 minutes; drain.
Place the remaining 3 tablespoons of butter into a slow cooker. Gently combine the pierogies, cabbage, and bacon; and place into the slow cooker. Cook on Low for 3 hours before serving.