Stir together the mayonnaise, sour cream, vinegar, onion powder, garlic powder, 1/2 teaspoon salt and a generous amount of pepper in a small bowl.
Put the lettuce in the bottom of a clear bowl or trifle dish, then layer the bell peppers and cucumber on top. Spoon the ranch dressing on top and spread to cover. Top with the scallions, followed by the pumpkin seeds and then the radishes. Cover with plastic wrap and refrigerate for up to overnight. Toss before serving.
Or if serving right away, toss immediately and serve.