Easy Pot Roast

I discovered this dish from my sister, who prepared it when we visited her in Florida last April.

1 pot roast, lean (chuck, rump, brisket, shoulder, tip or bottom round), about 3-4 lbs
12 cloves garlic, crushed
2 tablespoons oil
1 can (10-3/4 oz) cream of mushroom soup
Portabella mushrooms
1 onion, cut into rings
1 cup water (or milk), divided
6 medium potatoes, peeled and quartered
6 carrots, sliced in 2" pieces
2 tablespoons flour mixed with 1/4 cup cold water (optional)

Insert crushed garlic into the meat by puncturing it with a sharp knife. In a large pan, brown roast in oil; remove roast and set aside. Pour off excess oil. In the same pan, combine mushroom soup and 1 cup water. Heat to boiling. Return roast to pan. Reduce heat to low. Cover and simmer for 2 hours, turning roast occasionally. Add potatoes, carrots and portabella mushrooms. Cover and cook for another 40 minutes, or until vegetables are tender. If sauce has not thickened, remove meat and vegetables to serving platter. Gradually stir the flour and water mixture into the sauce in pan, stirring until sauce boils and thickens. Serve thickened sauce over roast and vegetables. Serves 6-8.