I accidentally bought tomatoes that were in tomato "sauce", which was similar to spaghetti sauce but without seasoning. I actually really liked the result this yielded. I also accidentally bought frozen spinach (stuuuupid), which made things difficult, but it still worked.
I see others used tomato paste, which is pretty similar to what I ended up doing. I'll definitely make it again with the right kind of spinach, use the same tomatoes I used last time, and add more spices. I like the idea of adding some meat, too.