Preheat oven to 375 degrees.
In a medium bowl put the potatoes, 6 tablespoons of the pesto and 1 ¼ cups of the mozzarella cheese and toss until well mixed. Set aside.
In a small dish put 1 tablespoon of the pesto and ¼ cup of the cheese and toss well. Set aside for part of the garnish.
In a medium skillet cook the sausage until cooked through, browned and crumbled. Pour off grease into a small heat safe dish and set aside. Add the jar of spaghetti sauce and the remaining 6 tablespoons of the pesto and mix well with the meat mixture. Bring to a simmer stirring and remove from heat. Reserve ½ cup of the meat sauce for the garnish.
In a 9x9 inch square baking dish pour one teaspoon of the reserved sausage grease into the dish. Brush sides and bottom of dish well with the grease. Add more if necessary to cover the dish. Set dish aside.
Pour the meat sauce and 2 Tablespoons of the remaining leftover sausage grease over the bowl of potatoes and toss mixing well until the potatoes are all covered.
Pour half the potato mixture into the prepared baking dish. Smooth out. Sprinkle 1 ¼ cups of the cheese over top. Pour remaining potato mixture on top of the cheese and again smooth out. Cover dish tightly with foil and bake for 30 minutes. Remove from oven and sprinkle the top with the remaining 1 ¼ cups cheese and put back into the oven UNcovered for 10 minutes until cheese is melted.
Remove pan from oven. Cut into desired amount of pieces. Put about one tablespoon or more of the reserved meat sauce on top and in the center of each piece as a garnish. Then divide evenly the reserved pesto/cheese mixture and put in center of the sauce on each piece to finish as the garnish. Serve hot. Makes 4-6 servings.