Heat the EVOO in a saucepot and add the butter. Add the garlic and cook, stirring, for 2 minutes. Add the tomatoes and crush them up a bit. Add the passata, a fat handful of torn basil leaves, the onion and some salt and pepper. Bring to a simmer and simmer for 40 to 45 minutes. Remove the onion and set the sauce aside.
Separate the spinach while adding it to a mixing bowl. Add half of the mozzarella, the ricotta, 1 1/2 cups of the Parmigiano-Reggiano and the eggs yolks and mix to combine. Season with grated nutmeg, salt and pepper.
Layer a little sauce into a lasagna pan and top with three flat lasagna sheets or four curly. Top with a little more sauce, and then half of the spinach and cheese mixture. Repeat with another layer, and end on top with lasagna sheets. Top with some sauce and the remaining mozzarella and Parmigiano-Reggiano.
The lasagna can be stored before or after baking for a make-ahead meal, or baked to serve immediately. When baking the first time, bake at 375 degrees F, uncovered, from room temperature until cooked through and brown and bubbling on top, 60 to 70 minutes. If reheating, bake at 375 degrees F, covered, for 40 to 45 minutes.
Let the lasagna rest 20 minutes before cutting. Serve with extra sauce on the side.