In a medium bowl, stir together the red onion with the lime peel, the 1/2 cup juice, and the 2 teaspoons sea salt to coat. Let stand for 30 minutes at room temperature or up to 24 hours in the refrigerator before serving.
In a small bowl, toss almonds with the 3 tablespoons lime juice, chili powder, the 1/4 teaspoon sea salt, and red pepper. Evenly spread almonds in a foil-lined 15x10x1-inch baking pan. Bake in a 350 degree F oven for 10 to 15 minutes, stirring every 5 minutes, or until toasted. Remove from oven. Cool.
To serve, place spinach on a large platter; drizzle with balsamic vinegar and oil. Top with grapefruit and avocados. Drain onions; discard liquid. Top spinach with pickled onions. Sprinkle with spiced almonds. Makes 8 servings.