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Heat olive oil in a large skillet over medium heat for two minutes. Add onions, sauté until translucent, about 5-7 minutes. Add garlic and sauté for two minutes.
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Reduce heat to medium/low. Add in the stock and diced tomatoes. Stir to combine. Layer halved lasagna noodles on top of the tomato mixture, gently pressing them in. Top the noodles with spinach, crushed tomatoes, and then the marinara. Season the top with the salt, pepper, crushed red pepper, and oregano. Gently spread the sauce and tomatoes all over the noodles. Cover with a lid and let simmer for 15 minutes.
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After 15 minutes, remove lid and poke the noodles around a bit. Gently stir the spinach into the sauce and lift some of the noodles so the sauce goes underneath them to prevent sticking or burning. Cover and let simmer for another 10 minutes.
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Add the ricotta to the sauce in several small dollops. Cover with lid and let the ricotta warm. After two minutes, sort of poke the ricotta under a few of the noodles to spread it out a bit. Turn the heat down to low.
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Add the slices of fresh mozzarella to the top and cover with lid until the cheese melts, about 3 minutes. Shut off heat and let the dish stand for 5-10 minutes before serving.