Preheat oven to 375 degrees. Heat heavy large saucepan over medium heat. Add sausage, onion, and garlic, and saute until sausage cooked through, about 10 minutes, breaking up meat with back of spoon. Add tomatoes with juice to pan. Simmer until sauce thickens slightly, stirring occasionally, about 10 minutes. Stir in pesto sauce. Season with salt and pepper.
Lightly oil 13 x 9 glass baking dish. Combine pasta, spinach, mozzarella, and 1/3 cup Parmesan cheese in large bowl. Stir in hot tomato sauce. Mix gently until spinach is wilted. Transfer mixture to prepared baking dish. Sprinkle remaining 2/3 cup Parmesan cheese over top. Bake until sauce bubbles and cheese melt, about 30 minutes.