Put the potatoes in a medium pot of cold salted water. Bring to a boil over medium heat and cook until fork-tender, about 15 minutes. Drain and rinse under cold water; transfer to a large bowl.
Pulse 1/2 cup gorgonzola, the parmesan, yogurt, garlic and 1/4 cup water in a blender or mini food processor until smooth; season with salt and pepper. Add half of the dressing to the potatoes along with the salad blend, Peppadews and parsley; toss.
Arrange the roast beef on four plates. Serve the salad on the side, topped with the remaining dressing and 1/2 cup gorgonzola.