Preheat oven to 425 degrees
In a large skillet cook onion and garlic in the 1 TBSP hot olive over medium heat about 4 minutes or until tender. Remove from heat. Stir in spinach, pine nuts, Asiago cheese, lemon peel, and salt and pepper to taste. Set aside.
Butterfly the flank steak by cutting it in half horizontally to, but no through, the opposite side, opening the steak like a book. Place the steak between two pieces of plastic wrap. Using the flat side of a meat mallet, pound steak lightly into a rectangle about 1/2 inch thick. Discard plastic wrap.
Spread spinach mixture evenly over flank steak to within 1 inch of edges. Layer cheese and then prosciutto over top. fold short ends of steak just over filling. Starting from long edge, roll up steak into a spiral, making sure the ends stay tucked in. Using kitchen string, tie roll at 2 inch intervals. Place meat on a rack in a shallow roasting pan. Brush roll all over with 2 TBSP olive oil and season with additional salt and pepper. Insert an oven meat thermometer into the center of the roll.
Roast, uncovered for 55 min or until temperature is 145 for medium-rare. Cover with foil and let stand for 15 minutes, internal temp should be 160. Slice meat into 1/4 inch thick slices.
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