Step 1

Set oven rack 5-6 inches from top and preheat broiler. Line a baking sheet with aluminum foil.

Step 2

Make vinaigrette by combining olive oil, balsamic vinegar, cinnamon, nutmeg and salt in a small bowl.

Step 3

Dip onions into vinaigrette, being careful to keep rounds intact, and lay in a single layer on prepared baking sheet. Broil until soft and caramelized, 5-10 minutes. Keep a close eye on them so they don't burn. Set aside or refrigerate until ready to assemble salad.

Step 4

Peel the oranges with a sharp knife (it's easiest to use a serrated knife), carefully removing all of the white membrane. Slice into rounds about ¼-inch thick, remove any seeds, and place in a 9x13-inch baking dish. Pour vinaigrette over top, making sure oranges are well coated on both sides, and let marinate in the refrigerator for at least 30 minutes or up to several hours.

Step 5

To assemble the salad, place spinach on large platter or individual plates. Scatter caramelized onions over spinach, and then place marinated oranges on top. Quickly whisk vinaigrette from soaked oranges, then drizzle over top or serve on the side.