I tend to use very little salt in cooking, and wouldn't add any directly to this recipe (and there would be none at all added to the tomatoes). There are a few things you could add that would contain a little salt (such as Worcester sauce, marmite, or even soy sauce - be sparing if you use any of these so they don't take over the flavour). What they have in common is umami. That link has a list of foods high in umami; one that stands out here is mushrooms.
There are other things you can do though. More onion would help, as would more herbs and black pepper. This recipe might benefit from some red wine, in place of any water you might add during simmering. A little celery or bell pepper can help a lot, even dried and powdered. Vegetable stock powder is a good source if you can get it without salt (I can). You can buy seasoning that's made mainly from powdered dried onion, garlic, celery and peppers with no added salt (though the one I have - "season-all" - might come in a little sweet for your use). Lemon juice can also help.
Personally I'd use several of the things I list, to avoid any single one dominating.