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In a large ovenproof skillet or pan, heat your olive oil on medium-high.
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Add the chopped onion (and garlic, if using) and sauté 2-3 minutes.
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Add the beef, oregano, and Italian seasoning and cook 3-5 minutes, until the beef is crumbled.
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In the same pot, combine your stock, tomatoes, salt, pepper, and red pepper flakes. Bring the mixture to a boil.
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Cover, reduce the heat to a simmer, and cook for 12-15 minutes, or until your pasta is done (do not over cook!).
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Stir in the spinach until just wilted.
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Sprinkle with mozzarella cheese and place under the broiler for 1-2 minutes, until the cheese is just melted.
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Serve and enjoy!