Select a few nice mushrooms for garnish. Trim stems. Wash, trim and chop remaining mushrooms.
Reserve 2 tablespoons of the chopped basil. Combine the remaining chopped basil, chopped mushrooms, onion, 1/4 tsp of the salt and pepper in a bowl.
Unroll the pork roast; lay flat on the counter. Starting in the center of the meat and holding a knife parallel to the counter, cut thick part of the loin through the center and almost in half. Fold the piece back like a book to increase size of the loin by 1/3.
Spread the basil-mushroom mixture over the meat. Reroll. Tie at 3 inch intervals with string. Place in a shallow roasting pan. Sprinkle remaining salt over top. Pour 1/4 cup of the chicken broth around roast. Refrigerate remaining broth.
Roast pork in the preheated oven for 2 1/2 hours or until a meat thermometer registers 170°F.
Just before the roast is done, saute reserved mushrooms for garnish in the butter in a heavy saucepan until golden. Remove mushrooms; set aside.
Reserve 1/2 cup of the refrigerated chicken broth. Pour remaining broth into the saucepan in which mushrooms were sauteed. Bring to boiling on top of stove.
Remove roast to a warmed serving platter; keep warm. Pour off fat from the roasting pan. Add hot chicken broth to the pan. Stir, scraping up browened bits form the roasting pan. Bring to boiling on top of stove.
Meanwhile, stir flour into remaining 1/2 cup chilled chicken broth until smooth. Gradually add to boiling broth in the roasting pan. Stirring rapidly with a wire whisk. Cook until smooth and thickened. Fold in reserved 2 tbsp chopped basil. Spoon some gravy over roast. Pour the remainder into sauce boat. Garnish roast iwth reserved mushrooms and fresh basil leaves.