Spinach & Four-Cheese Stuffed Shells with Marinara Sauce

I got this craving for a stuffed pasta dish, so I hunted all over the internet for a recipe I liked. I found one that looked easy (enough) and delicious, so I thought I'd try it on my next day off. The gal who submitted the recipe also had a recipe for the sauce, so I tried that as well. And although I bought manicotti, I was not confident enough that I could stuff them properly, so I chose to cook the giant shell pasta. Our friends came over and we devoured 95% of it! Anyways, it was yummy, and I'll make it again, with a few minor tweaks, I'm sure.

First up, the marinara sauce.
3 - 4 Tbl. olive oil
1 large onion, chopped
4 - 6 garlic cloves, minced
1 Tbl. dried basil
2 tsp. dried oregano
1/2 tsp. crushed red pepper flakes
1 (6 oz.) can tomato paste (can use two for thicker, richer sauce)
1 (28 oz.) can whole tomatoes, undrained and broken up with a fork before cooking
2 tsp. sugar
1 tsp. salt (I used more)
fresh ground black pepper to taste

1. Heat oil over medium-high heat.
2. Add in onions, garlic, dried red pepper flakes, basil and oregano; saute for 3 - 4 minutes.
3. Add in tomato paste and stir for 2 - 3 minutes.
4. Add in the tomatoes with their juice; simmer uncovered over low heat for 1 - 2 hours, stirring occasionally.
5. Season with salt, pepper and sugar.
(for a thinner sauce, add in some water)

stuffed pasta

3 Tbl. oil
1/4 c. onion, finely chopped
2 tsp. fresh garlic (about 2 cloves)
1 1/2 c. ricotta cheese
1 1/2 c. shredded mozzarella cheese (can use a bit more)
4 oz. cream cheese, softened
1/3 c. grated parmesan cheese (can use a bit more)
1 tsp. italian seasoning
salt to taste
1/2 - 1 tsp. fresh ground black pepper (or to taste)
1 (10 oz.) package frozen chopped spinach (cooked according to package directions, then hand-squeezed dry to remove excess moisture)
10 manicotti (or 3/4 box large shells), cooked and drained
4 - 6 c. marinara sauce (see above)
1/4 c. parmesan cheese
mozzarella cheese, for topping

1. Preheat oven to 350 degrees.
2. Butter a 9" x 13" baking dish.
3. In a skillet, saute onion in oil for about 4 minutes or until softened (add in garlic the last 2 minutes).
4. In a mixing bowl combine ricotta cheese, 1 1/2 c. mozzarella cheese, cream cheese, 6 Tbl. parmesan cheese, italian seasoning; beat until smooth and well combined.
5. Stir in onion mixture and spinach, season and mix well to combine.
6. Spoon into cooked and cooled shells.
7. Pour half of the pasta sauce into the baking dish.
8. Arrange shells over sauce, top with remaining sauce.
9. Sprinkle about 1/4 c. parmesan cheese (or less) over the sauce.
10. Cover and bake for 25 minutes.
11. Uncover and top with mozzarella cheese, bake 5 minutes longer, or until cheese is melted.
12. Let stand 10 minutes before serving.