Is this…an actual thing people make? Certainly I've never seen it, and Googling "fish steak casserole" or "fish potroast" and similar isn't coming up with anything. It's from a late 70s-early 80s cookbook series, if that helps. Tuna was probably cheaper then.
The recipe is pretty much how to get to this…thing:
From a 3-5 lb tuna steak and some veggies:
(yes it is being larded with anchovies)
Here's the whole process:
My conviction that this is a horrible thing to do to a nice piece of fish is balanced by a desire to know what it tastes like. I mean, all the ingredients are good. It just looks like a dog vomited on a giant can of tuna and tried to cover it with nearby foliage.