Roast the potatoes:
Heat the oven to 400°F. Put the potatoes on a baking sheet and roast until tender when pierced with a fork, about 1 hour. Let cool, peel, and break into 1- to 2-inch chunks. Put them on the baking sheet and set aside.
Make the vinaigrette:
Heat 2 Tbs. of the olive oil in a sauté pan over medium heat. Add the shallots and sauté until softened, about 1 min. Whisk the vinegar and mustard in a medium bowl. Whisk in the remaining 1-1/2 cups oil. Add the sautéed shallots and their oil, along with the thyme. Season with salt and pepper to taste.
Fifteen minutes before serving:
Heat the oven to 375°F. Put the walnuts on the baking sheet with the potatoes; warm in the oven until the potatoes are hot, about 10 min. Put the potatoes and walnuts in a large bowl. Stir the vinaigrette; pour 1 cup of it over the potatoes and walnuts (save the rest to drizzle over the beef tenderloin). Let sit for a couple of minutes. At the very last minute, add the Gorgonzola and spinach. Toss well but gently; don't mash.
nutrition information (per serving):
Photo: Scott Phillips