Where can I find two roasting pans that attach together?
 

This used to be common place when I worked as a cook (20 years ago now!!!). The equivalent of a roasting pan and a baking tray which interlocked so that you could steam things like fish inside. Have looked in practically every shop in the South of England, but they don't seem to make these anymore:chef:

Like this:

https://ecx.images-amazon.com/images/...L._AA1500_.jpg

Amazon.com: WMF Vitalis Stainless Steel Large Stovetop Steamer System, 16-Inches by 11-Inches: Kitchen & Dining

There are a lot more round ones and a lot of round steamer inserts for round pots, but I thought this might be more what you had seen before. Hopefully, it can be found less expensive.

Here's something similar:
Amazon.com: Large Deep Steamer 14 Inch Skillet Frying Pan: Kitchen & Dining

And there are a lot of fish poachers around.
https://www.cookware-online.co.uk/ima.../fishpoach.jpg

Kitchen Craft 24'' Fish Poacher - Cookware Online