Fill a large pot half full with water and bring to a boil. Meanwhile, trim the greens and rinse well in cold water.
Working in 3 batches, cook the greens in the boiling water until just wilted, about 1 minute per batch. Using tongs, transfer the cooked greens to a colander. Press on the greens to drain excess water (do not squeeze) coarsely chop and set aside.
Sprinkle the onion and bacon in the bottom of a large, lined slow cooker. Add the chopped greens; season lightly with salt and pepper. Cover and cook on high until the greens are very tender, about 2 1/2 hours. Season before serving.