I make my sauce from tomato juice. You do not need to add flour. Here is what I do: Cut up onions, garlic, celery put in large saucepan with your tomato juice. I use two cans. Then I add in spices - basil, oregeno, parsley, Italian seasoning, whatever you like. Then I let it cook on low for hours. I just keep checking on it. Stir. Be sure it does not stick. About an hour or so before serving, I check on sauce taste, usually need to sweeten a little. I use splenda for this. I also sometimes add in a little Parm. cheese and if I think sauce is too thin, I add in a little Italian flavored bread crumbs. Just watch it closely after this as last two need to be stirred and watched so doesn't stick. My crew loves this. The juice thickens with all the slow cooking. Good luck!
Pam