A description of the process for spinach is described
here.
I'd likely go with 2 bags pre-washed spinach, 3 if on the light side (spinach reduces dramatically when bled
or sautéed, as you know); a few tablesppon of freshly grated parm; enough of the softer cheese(s) (the fontina or fontina-mozz blend) to cover the interior well but not thickly; a medium nion, finely chopped; a couple or three minced cloves or garlic; 2-3 ounces thinly sliced prosciutto, finely chopped (par-freeze first to make this easier); 1/4-1/3 cup oil-packed sun-dried tomatoes, drained well, finely chopped; if you wish you can add an equal amount finely chopped peeled diced tomato, well drained.
If you go with the sun-dried use some of the oil from the jar to sauté the onion till just beginning to color; add the garlic and cook, stirring, till it's very fragrant, about 45 seconds - don't let it brown. Scrape all into and allow to cool some. Add the bled spinach to the bowl, breaking it up with your fingers as you do (it packs very tightly when squeezing it during the bleeding process). Continue breaking it up with a spoon once in the bowl so that the bowl mixture combines well.
If you've left the spinack lightly seasoned with salt, good. Stir in a little pepper to taste, the prosciutto, the sun-drieds, the fresh tomato (if using), and the parm.
Lightly salt and pepper the interior of the roast, topped with some of the frated cheese, spoon on the filling from the bowl, spreading it, add the reszt of the cheese, roll and tie. Season the exterior; cook.
