Combine the split peas and 2 cups of the water in a small saucepan over high heat, and bring them to a boil. Once boiling, lower the heat and cover to let the peas simmer until tender, about 25 to 30 minutes. Drain and rinse the cooked peas and transfer them to the bowl of a large food processor.
Use the same pot to cook your jumbo shells according to the directions on the package. (I like to do this while making the pesto, so that everything is ready at about the same time.) Preheat the oven to 350ºF and spread a bit of the marinara sauce at the bottom of a 9"x 13" glass baking dish. Set it aside.
To prepare the pesto, add the remaining ¼ cup of water, fresh basil, 1 tablespoon of lemon juice, 1 clove of garlic, and the salt to the food processor, and process it together with the cooked split peas. Taste the mixture and add more lemon juice or garlic if desired.
Once the shells are tender, drain them and fill each shell with a large spoonful of the pesto. Arrange the stuffed shells into a single layer in the prepared baking dish and spoon additional marinara sauce over the top of each shell. Bake until everything is heated through, about 25 to 30 minutes. Serve warm, garnished with fresh basil.