Remove coarse stems from greens and discard. Wash and spin dry, then cross cut into 1" square pieces.
Drop greens into a pot of boiling, lightly salted water. Cook uncovered 5-10 minutes, or until just tender. Timing will depend on the age and size of the greens. Very young greens will need only 1 or 2 minutes. Drain thoroughly.
Meanwhile, in a large non-aluminum skillet, melt butter and oil over medium heat. Add onion, stirring frequently, 5 minutes or until softened and slightly browned. Add black-eyed peas and drained greens. Toss to coat.
In a small bowl, combine sour cream, mustard (or horseradish if using), and cayenne pepper with a whisk or fork. Add to peas and greens and cook, stirring frequently, 3-5 minutes or until liquid has almost evaporated. Season with salt and pepper and serve warm. Have a little more sour cream/mustard (or horseradish) to serve on the side.