Set a large pot of salted water over high heat and bring to a boil. Preheat the oven to 350 degrees F. Once the water is boiling, add the pasta shells and cook for 6-9 minutes, to al dente. Drain the pasta and set the shells in a 9x13-inch baking dish.
For the Avocado Pesto: Place all the ingredients in a food processor. Pulse until the mixture comes together into a mostly smooth paste. If the pesto seems a little glumpy, add 1/4 water then pulse to combine.
For the Ricotta Filling: Mix the ricotta, 3/4 cup parmesan cheese, and salt together in a bowl.
Pick up each shell and gently squeeze the ends to open the shell wide. Spoon a heaping tablespoon of ricotta filling in each shell. Then spoon 1-2 tablespoons of avocado pesto into each shell, pushing the ricotta to the side. Brush water over the top of the shells in order to help the shell edges not get too crispy.
Bake the shells for 15-20 minutes. Then sprinkle 1/4 cup parmesan and 1/2 cup mozzarella over the top and bake another 2 minutes to melt. Serve warm!