I’m kinda new to this. What are “strained tomatoes”? Is that canned diced or stewed tomatoes, drained? Fresh tomatoes peeled and pushed through a...

You may also find Italian brands sold as "Passata," as in passed through a strainer.

This is a commercial tomato product that is cooked and strained to remove lumps and seeds.

I have used the following product for over two decades:

https://www.amazon.com/Pomi-Strained-Tomatoes-26-45-Each/dp/B002FBV2IQ

Pomi is a brand from Italian food giant Parmalat. Their products generally come packaged in a Tetra-Brik.

I don't buy these online since I'm able to find them at my local grocery stores.

There are likely competitors who offer comparable products but I have not researched them since I have been happy with the Pomi products for 20+ years although occasionally I'll buy San Marzano cans.

I'm sure you can make your own from scratch using fresh tomatoes (after all, that's how all these products start) but I would put my efforts into utilizing fresh tomatoes in different ways than to include them into a recipe like this one. But that's just me...

Anyhow, many of these products are pretty interchangeable since they will be cooked well. The main thing is whether or not you want to stay faithful to the author's preference for a smoother sauce with no tomato seeds.

Your call.

Best of luck.