My Mom's Tarragon Chicken

4 each chicken drumsticks and thighs (about 2 pounds) skinned (I, Nadia, just use one of the cheap whole chickens from Costco and cut up the pieces)
1 large onion
4 cloves garlic, peeled and thinly slices
2 plum tomatoes, diced (I usually don't have these so I use the canned diced tomatoes and drain the juice out of them)
1/4 c. tarragon vinegar (I have not been able to find this in NM so I substitute 1/4 cup white vinegar)
1 1/2 Tbs. chopped fresh tarragon or 2 tsp dried
1 Tbs. Dijon mustard
1/4 tsp salt
1/2 tsp pepper (I always forget this)
1/2 cup heavy cream (I have used milk when I did not have cream_
2 Tbs. all-purpose flour

1. Mix all ingredients except heavy cream and flour in large slow-cooker.
2. Cover and cook on high 4 hours of low for 7 hours until chicken is cooked through and onions are soft.
3 Whisk heavy cream and flour in small bowl until smooth. Stir in some hot liquid from cooker; stir into liquid in cooker. Turn setting to high; cook 25 minutes until liquid thickens.
4. Serve over rice