Preheat the ovento 350 degrees. In a large skillet add the beef, onions, garlic, salt and pepper into the pan and cook on high. Once browned, turn off the heat, drain the liquid left in the pan and mix in one cup of marinara sauce. Add the cooked spaghetti to the remaining 5 cups of the marinara sauce and toss to combine. Mix the cream cheese and 2 cups of mozzerella cheese in bowl. Add halg the pasta/sauce to the bottom of a 9×13 pan. Add the cream cheese mixture and top with the remaining cup of mozzerella cheese. Cover and bake for 30 minutes. Uncover and bake for an additional 10 minutes until cheese is melted and bubbly.