Mustard-Sauced Chicken

4 skinless, boneless chicken breasts (4oz each)
2 Tbsp butter
1 onion, chopped
1 (10oz) can cream of mushroom soup
2 Tbsp coarse-grained mustard
1/2 C plain yogurt
1/2 C water
1/4 tsp dried tarragon
1 1/2 C instant rice
1 tsp salt
1/2 C seedless red grapes, halved
2 Tbsp dried chives

Slice chicken. Heat butter in a large skillet over high heat; add onion and chicken. Cook until onion is golden, about 10 minutes. Stir in soup, mustard, yogurt, water and tarragon. Bring mixture to a boil. Stir in rice and salt; remove from heat. Let stand, covered, for 5 minutes. Stir in grapes. Spoon into serving dish; top with chives.