Braised Sweet and Sour Beef-Sirloin cooked in the oven with a delicious tangy sauce

This recipe is the blending of two into one.  A sweet and sour sauce, and a slow cooked sirloin recipe.  I served it with white rice.  Very easy recipe, and smells amazing in the oven! I happened to have sirloin, but would work equally well with chuck, or a 7 bone roast.  You might need to increase the cook time for a thicker cut of meat.  The sauce becomes sticky and mixes with the beef juices during the long cooking time, and the meat is very tender.  One of my family's new favorites.

Braised Sweet and Sour Beef
2 # sirloin steak
2 yellow onions thinly sliced
6 cracked cloves of garlic
pinch of red pepper flake
Preheat oven to 300 *F.  In a deep sided cast iron pan, sweat the onions, garlic and red pepper in oil on medium heat.  When they become soft and opaque, scrape the oil and onions mix into a medium sized mixing bowl. Place sirloin in pan you cooked the onions in.  
To the mixing bowl add :
1 12 oz can tomato paste
1/4 C brown sugar (I use splenda brown sugar instead)
1/3 C cider vinegar
2 C beef stock
Whisk together until combined and pour over steak.  Season with salt and pepper.  Bake for 2 hours.  In the last ten minutes, top steak with thinly sliced green pepper rings.  Serve meat and sauce with white rice.
Four large servings, or six normal ones