Place oven rack in the center position and preheat the oven to 375 degrees. Line a baking sheet with foil. Slice the potatoes lengthwise in half and then in half again so that you have wedges that are about 1/2 inch or more thick. Drizzle potatoes with olive oil, sprinkle generously with salt and pepper, then toss gently. Roast until golden and tender when pierced with a knife, about 20 minutes. Remove and cool for 3 to 4 minutes. (You can roast potatoes ahead and let them cool but they are best when eaten soon after they come out of the oven.) Transfer potatoes to a salad bowl and toss with 1/2 the dressing. Add spinach and toss again, adding just enough additional dressing to coat the leaves lightly. (You may not need to use all the dressing.) Taste and if needed season with more salt and pepper. Mound potatoes and spinach onto 4 salad plates and sprinkle each serving with goat cheese.