1) Preheat oven to 180C/Gas Mark 4.
2) Slice the onion into thin wedges, through the root end. Toss with the olive oil, season with salt, to taste, and spread out on a baking sheet. Roast until just soft and brown, about 20 minutes. Set aside to cool.
3) Put the spinach and onions in a large bowl. Drizzle with some of the dressing and toss. Add more dressing as needed; spinach should just be lightly coated. Divide salad among 4 chilled plates. Top with pecans, goat cheese, and dried cranberries. Serve immediately.
4) To make the citrus dressing: Whisk the lemon zest, lemon juice, orange juice, Dijon, honey, thyme, and salt in a medium bowl until well combined. Gradually whisk in the oil, starting with a few drops and then adding the rest in a steady stream, to make a smooth, slightly thick dressing.