Place an oven rack in the center of the oven. Preheat the oven to 375 degrees F. Line a heavy baking sheet with parchment paper. Set aside.
For the dressing: In a large bowl, whisk together the olive oil, lemon juice, mustard, thyme, salt, pepper and lemon zest until smooth. Remove 3 tablespoons of the dressing and reserve.
For the salad: Add the potatoes to the bowl and toss until coated with dressing. Arrange the potatoes in an even layer on the prepared baking sheet. Season with salt and pepper. Roast until golden and tender, about 40 minutes.
In a large bowl, toss together the reserved 3 tablespoons dressing, bell peppers and arugula. Arrange on a platter. Place the roasted potatoes on top and sprinkle with the gorgonzola cheese and chopped walnuts if using.
Cook's Note: To toast the walnuts, arrange in a single layer on a baking sheet. Bake in a preheated 350 degrees F oven lightly toasted, 6 to 8 minutes. Cool completely before using.