In the bowl of your food processor, place half the spinach and basil and pulse until broken down. Add the remaining spinach, and pulse until it is finely chopped.
Add the garlic, walnuts, salt and pepper and pulse until all the ingredients are finely chopped.
Scrape down the sides of the bowl, replace the top and start the motor. With the motor running. slowly drizzle the olive oil from the top until the pesto is smooth. Set aside.
Place paper towels down on a cutting board and the sliced tomatoes on top of it, sprinkle lightly with kosher salt and top with more paper towels.
Let the tomatoes sit for 5-10 minutes to drain excess moisture.
Pre-heat your oven to 350 degrees F.
In a 9X13 dish, spread 1/4 cup of the pesto evenly across the bottom.
Place one layer of lasagna noodles and then dollop 1/2 of the ricotta. Top with 1/3 of the tomatoes.
Add another layer of lasagna noodles and spread more pesto across it. Top with 1/2 of the fresh mozzarella and 1/3 of the tomatoes.
Add the next layer of lasagna noodles, more pesto, the remaining ricotta and tomatoes.
Place the final layer of noodles on top and finish with a little more pesto and the remaining mozzarella.
Bake, covered with aluminum foil for 30 minutes.
Remove the foil and sprinkle the top with the parmigiana cheese. Bake for an additional 5 minutes.
Remove from the oven and let sit for 5-10 minutes prior to serving.