Wash beans. Place in top half of steamer over boiling water and steam for 2 minutes or until crisp and tender. Do not overcook.
Remove from steamer and quickly shock under cold running water. When well chilled, place on a paper towel to drain.
Peel and remove any pith from oranges. Slice crosswise into very thin slices. Set aside.
Peel and trim onion. Slice crosswise into very thin slices. Set aside.
Combine oil, vinegar, mustard, orange juice, maple syrup, mint, salt and pepper in food processor fitted with the metal blade; process for 30 seconds or until well combined. Taste and adjust seasonings.