With vegetable peeler, peel off four 2 1/2x2-inch strips of orange peel, being careful to remove only orange portion. Cut strips lengthwise into thin slivers.
With sharp knife, cut remaining peel and white pith from oranges. Remove segments of orange by cutting between membranes, catching juice in small bowl; set orange segments aside. Squeeze membranes of oranges into bowl to remove all of juice. If necessary, add orange juice to make 1/2 cup. Add honey, vinegar, oil and salt; beat well with wire whisk.
Place onion slices in single layer on cookie sheet; brush with orange juice mixture. Broil 2 to 3 inches from heat for 4 to 5 minutes or just until edges begin to darken.
Arrange spinach on serving platter or line individual salad plates with spinach leaves. Arrange orange segments on top of spinach. Separate onion slices into rings; scatter over oranges. Drizzle with remaining orange juice mixture. Garnish with slivers of orange peel.