Pass the tomatoes and their juices through a food mill or break them up by hand. If you break it up by hand remove the stem cores and any skin.
Heat a saucepan large enough to hold the sausage and tomatoes over medium heat.
Add enough olive oil to film the bottom of the saucepan.
Crumble the sausage into the pan and cook until just done. Remove, using a slotted spoon.
Spoon off all but 2 Tbsp of fat. If you don't have 2 Tbsp check the bottom of the bowl you put the cooked sausage into. It's there waiting for you.
Turn the heat down to medium low and add the onions.
Cook, stirring occasionally, until the onions are soft and translucent. Do not let them brown.
Add the garlic and cook for 30 seconds, stirring constantly.
Hopefully cooking the sausage left a bit of browned goodness in the bottom of the saucepan. If it did, pour about 2/3 of a cup of the crushed tomatoes into the saucepan, bring to a boil and scrape it up. If you didn't get any browned goodness just add all the tomatoes and next time cook the sausage over higher heat.
Return the sausage to the pan.
Add any remaining crushed tomato.
Simmer for 30 minutes or more.
Taste and adjust for salt. It likely won't need much, if any. This depends on how salty your sausage is.
This sauce is great with any pasta.