Combine bulgur and boiling water in a large bowl. Cover and let stand 30 minutes, and drain.
Peel and section 2 oranges over a bowl; squeeze membranes to extract juice. Set sections aside; reserve 1/4 cup juice. Discard membranes. Combine reserved orange juice, sugar, oil, salt, and pepper; stir well with a whisk.
Add orange sections, parsley, mint, cucumber, and onion to bulgur mixture; stir well. Add orange juice mixture; toss gently to coat. Cover and chill at least 2 hours.
Arrange the orange slices on each of 5 serving plates; top with tabbouleh. Sprinkle evenly with sunflower seeds.