Heat large Dutch oven over med/high heat and add the sausage.
Cook, breaking up the meat with a spoon for about 5 min or until
no pinkness remains. Add the onion and sauté for 5 min. until
softened. Add garlic and cook 1 min. Add the marinara sauce and
reduce heat to medium. Simmer for 10 min, or until sauce start to
thicken. Stir in the pesto. salt and pepper. Taste to adjust
seasonings. Set side.
In a LARGE ( I mean Large!) bowl, combine the pasta with the
sauce, diced cheese, 1/3 cup parmesan,and the spinach leaves,
mixing well to combine. Spoon mixture into prepare dish and
sprinkle with remaining 2/3 cup of parmesan.
Bake for about 30 min or until casserole begins to bubble and
cheese is browned.