For the pesto: Put the spinach, basil, nuts, olive oil, Parmesan, garlic and lemon juice in a food processor and pulse until well combined. Transfer to a bowl and stir in some salt and pepper.
For the pasta: Preheat the oven to 375 degrees F. Bring a large pot of salted water to a boil over high heat. Add the shells and cook until al dente, about 10 minutes. Drain and set aside, making sure to reserve a cup or so of pasta-cooking liquid.
While the pasta cooks, mix the ricotta with 2 tablespoons of olive oil and some salt and pepper in a small bowl. Mix the breadcrumbs with the remaining 2 tablespoons of olive oil, the Parmesan cheese, lemon zest and some salt and pepper in another small bowl.
When the pasta is ready, put it in a large bowl and mix in the pesto, tomatoes, and enough reserved cooking liquid to make a sauce. Transfer the mixture to a 9-by-13-inch baking dish, dollop with the seasoned ricotta and top with the breadcrumb mixture. Bake until golden and bubbly, 25 to 30 minutes. Let stand for 5 minutes before serving.