When it says you can prepare fish using Marcella Hazans recipe, does it mean you cook the fish in the oil first then add the sauce? Or are you act...

Heat the sauce, then add the fish fillets to the pot. Poach at a simmer, flipping halfway through, until fillets are cooked.
Alternatively, place fillets in a baking dish and cover with the sauce. Bake at 300 for about 20 minutes, or until cooked, depending on thickness.

I've done it both ways. Pan fried the fish so it had a nice little crust and then placed it on top of the sauce (or visa versa) and I've poached the fish in the sauce as CV suggested. Both are equally as good.

Hi Sal, I'm guessing you're referring to this article, right? https://food52.com/blog/16445-how-to-make-a-week-of-meals-with-marcella-s-genius-tomato-sauce You could go either way, but the method described here is to poach it directly in the sauce, which is similar to another Marcella Hazan technique for "Fish in Crazy Water" http://www.foodandwine.com/recipes/fish-in-crazy-water

If you are referring to this post:

https://food52.com/blog/16445-how-to-make-a-week-of-meals-with-marcella-s-genius-tomato-sauce

then you are supposed to poach the fish fillets in the tomato sauce itself. The tomato sauce acts as a poaching liquid, just like water, stock, milk, wine, etc. Thin down the sauce enough and you'll just have fish soup.