Lightly grease a 10-inch square of aluminum foil with butter or vegetable oil and set aside.

Combine 3 tablespoons sugar and 1 tablespoon water in a small saucepan and cook over medium-high heat, swirling occasionally, until the sugar turns a golden amber color, 3 to 4 minutes. Add the almonds and stir to coat. Continue cooking until the almonds are fragrant and golden brown, about 1 minute. Transfer to the prepared aluminum foil, using a spoon to spread into a thin layer, and set aside to cool completely.

With a thin, sharp knife, cut the peel and bitter white pith from the oranges, 1 at a time. Working over a bowl to catch the juices, cut in between the membranes to release the segments. Reserve the segments in a separate bowl. Combine 1/4 cup orange juice, the remaining 3 tablespoons sugar, the Champagne vinegar, olive oil, orange zest, 1/4 teaspoon salt, and the cayenne in a mixing bowl and whisk to blend. Transfer the dressing to a plastic container with a tight-fitting lid until you are ready to serve.

Put the spinach in a large serving bowl, then top with the orange segments, celery, and red onions. Cover with plastic wrap and refrigerate until ready to serve.

When ready to serve, break the caramelized almonds into bite-size pieces and scatter over the top of the salad. Drizzle the dressing over the salad and season with salt and black pepper, to taste. Toss to coat evenly and serve immediately.