Meanwhile, we start on our filling. In a large saute pan we heat ½ tablespoon of olive oil over medium-low heat. The mushrooms are sliced and added to the hot pan in a few batches as the cutting board is filled with sliced mushrooms. When all the mushrooms are added, we raise the heat to medium and stir occasionally. The mushrooms are done when they have released their liquid, this takes about 5 minutes. Salt and pepper are added and the mushrooms are spooned into a medium sized bowl.
To the same pan, another ½ tablespoon of oil is added and the heat is reduced to medium-low. We add our onions to the pan and cook, stirring occasionally, for about 5 minutes. We want them tender and golden. While the onions are cooking, the zucchinis are cut lengthwise in half, each half quartered, and then sliced, yielding 1½ cups. Next, the garlic is peeled and minced with a little kosher salt. When the onions are golden, the zucchini is added, and the heat is raised to medium. The zucchini is cooked for about 5 minutes until tender. We add salt and pepper and spoon the cooked veggies into another medium bowl. There are more mushrooms than zucchini – we could have combined them together and divided them evenly in the 3 layers of lasagne. Instead, we will have 2 layers of zucchini and 3 of mushrooms.
We add the rest of the olive oil to the pan along with the minced garlic. When the garlic starts to sizzle, we open the bag of spinach and dump it in. Stirring often, the spinach is wilted and then salt and pepper are added. We turn off the heat and pour the cooked spinach onto a large plate to cool.
When the water is boiling for the pasta, we add the noodles from the package. We stir and cook for about 9 minutes. The noodles are drained when they are a little harder than just al dente. Having drained the noodles, we spread them in a single layer on wax paper with a pair of tongs.
Meanwhile, we combine the cheeses. Over a piece of wax paper, we grate the mozzarella using our box grater. We put the shredded mozzarella into a large bowl, reserving ¼ cup for the top of the lasagne. The parsley leaves are quickly chopped and added to the shredded mozzarella with the Locatelli and ricotta cheese. We add the cooled spinach mixture, some salt and freshly ground black pepper, stir, and taste for seasoning (it’s delicious!). At this time, the beaten egg is added and combined.
We coat the bottom of a 13 × 9 inch baking pan with some tomato sauce. Arranging 4 pieces of lasagne lengthwise, overlapping the edges slightly, we begin the layering. The noodles are topped with a third of the spinach-ricotta filling, a third of the mushrooms, and half of the zucchini. We sprinkle a ½ cup or so of sauce over the veggies and cheese.
We add more lasagne, perpendicular to the first batch, cutting off the couple of inches of noodle that does not fit with scissors (that’s how Robin’s grandmother always did it). The trimmings are set aside.
The next layer is topped with another third of spinach-ricotta, another third of mushrooms, and the rest of the zucchini. More sauce is added, followed by another 4 pieces of lasagne lengthwise. The remaining fillings are spread on and topped with sauce.
We lay the final layer of noodles crosswise. We spoon sauce to cover and sprinkle on the reserved mozzarella. The trimmings and the few long pieces of lasagne that were not used are re-boiled for the children and served with tomato sauce
We place the pan, covered with foil, into the oven and bake for 30 minutes. When it is bubbling, we remove the foil and set aside, baking the lasagne for another 10 minutes.
We remove the bubbling pan from the oven, cover loosely with the foil, and allow it to rest for 10 minutes. The lasagne is served with additional hot tomato sauce.