I am asking for advice on canning a tomato pasta sauce. It consists of canned tomatos, butter and some onion.

I already did a Google search, but the results contradict each other. Some people claim that you can just pour the hot tomato sauce in a cleaned glass, put the lid on & flip it over until cool. Another method seems to involve cooking the glasses in hot water (stovetop or oven). But the reason that really leaves me unsure is an article that claims that you can't can a tomato sauce at all without a pressure canner because of the pH-value.

So what is this all about - can I safely can that sauce, and if so, how? How long can I store the sauce?

I am asking for advice on canning a tomato pasta sauce. It consists of canned tomatoes, butter, and some onions.

I already did a Google search, but the results are contradicting. Some people claim that you can just pour the hot tomato sauce in a clean glass, put the lid on & flip it over until cool. 

Another method seems to involve cooking the glasses in hot water (stovetop or oven). But the reason that really leaves me confused is an article that claims that you can't can a tomato sauce at all without a pressure canner because of the pH-value.

So what is this all about - can I safely can that sauce, and if so, how? How long can I store the sauce?