Rinse the mustard and turnip greens thoroughly to remove gritty dirt, and place in a 12 quart pot with 8 cups of water. Add the butter, pickle juice, bacon and onion, and bring to a rolling boil.
Continue cooking at a full boil for 20 minutes, then reduce the heat to low, and simmer for an additional 30 minutes, until tender. Taste and season with salt and pepper.
