For the tomato sauce:
- Add olive oil to a heavy-bottomed pot over medium-high heat. Add onion and sauté until browned.
- Add tomato puree to pot. Season with salt and stir in. Bring to a boil, then reduce heat and simmer for three to four hours.
Preheat oven to 475 F (250 C).
For the meat sauce:
- Heat olive oil in a pan over medium heat, then add green onions and sauté until softened.
- Add ground veal, season with salt and pepper, and cook. Once veal is cooked through, remove from heat and set aside.
For the lasagna:
- Boil a large pot of salted water. Add a little olive oil to the water. Add sheets of lasagna to the pot of water one by one, then boil for two to three minutes.
- Carefully remove the sheets from the pot and immediately put them in bowl of ice water to stop them from cooking.
To assemble the lasagna:
- Take a lasagna pan and add a scoop of tomato sauce to cover the bottom of the pan.
- Lay down sheets of lasagna over tomato sauce to completely cover the bottom, cutting the noodles to fit if necessary.
- Layer over a hefty dose of grated Pecorino, ground veal, and mozzarella to completely cover the lasagna noodles.
- Add another layer of lasagna sheets and repeat until you run out of lasagna sheets or meat sauce. Top the last layer of noodles with tomato sauce and Pecorino only.
Put in the oven to bake. When you see the lasagna noodles have started to cook, lower the heat to 300 F (150 C) and leave it for an hour and a half, periodically checking that the sauce hasn’t dried up, and adding more if it has.