Peas straight from the freezer retain their crisp exterior and soft, creamy interior in this simple-to-make salad. For a subtler onion flavor, soak onion slices in cool water, then thoroughly drain before folding into the mayonnaise mixture.
Whisk together mayonnaise, sour cream, and vinegar in a large bowl; season generously with salt and pepper. Fold in peas, onion, and parsley. Refrigerate at least 4 hours.
Meanwhile, cook bacon in a large skillet over medium heat until browned and fat is rendered, 8 to 10 minutes. Transfer to paper towels to drain and crisp.
Let salad stand at room temperature for 10 minutes. Gently fold in cheddar and bacon just before serving.