Preheat oven to 350 degrees. Spray a 9X13 inch baking dish with nonstick cooking spray; set aside.
In a large pot over medium-high heat, add the beef and sausage and sprinkle with salt and pepper. Brown mixture, crumbling into large pieces. Transfer to a paper towel-lined plate. Discard all but 2 tablespoons of drippings from pot. If you don't have that amount remaining, add olive oil to make up the difference.
Add the onion, garlic, and 3 teaspoons Italian seasoning and saute, over medium-high heat, until soft, about 5-7 minutes. Return beef and sausage to the pot. Add half of the marinara sauce (2 cups or 1 jar), water, and heavy cream to the onion and meat, turn the heat to low and heat through; set aside.
Meanwhile, bring another large pot of water, with 1 tablespoon of salt, to a boil. Cook ziti according to the package instructions. Drain well, then return pasta to original pot off the heat, and add the remaining half (2 cups or 1 jar) of marinara sauce to pasta and combine; set aside.
In a medium bowl, stir together ricotta, egg, remaining 1 teaspoon Italian seasoning, and nutmeg; set aside.