Warm Sweet Potato and Green Bean Salad

Ingredients

• 2 tbsp vegetable oil
• 3 medium sweet potatoes, cooked, pared and cut into 1/4-inch slices(2 lb)
• 1/2 lb. fresh whole green beans
• 1 small purple onion, halved and sliced
• 1 medium clove garlic, minced
• 1 cup watercress sprigs
• 1 tbsp red wine vinegar
• 1/2 tsp salt
• 1/4 tsp freshly ground black pepper