1. Preheat oven to 350ºF. Spread almonds on a baking sheet. Toast in oven, stirring once or twice, until light golden, about 8 minutes. Transfer to a bowl.
2. Place spinach and nectarines in a large bowl. In a small bowl, stir together vinegar and sugar until sugar dissolves. Season with pepper and a pinch of salt.
3. Warm olive oil in a large skillet over medium heat. Add onions and cook, stirring frequently, until well browned, 10 to 12 minutes. Stir in vinegar mixture and remove from heat.
4. Scrape hot dressing over spinach mixture and toss to coat. Divide among 4 dinner plates. Sprinkle each salad with goat cheese, almonds and ham, if using. Serve immediately.