Bring a large pot of salted water to a boil. Add 6 oz. (170 g) jumbo pasta shells and cook for 2 minutes shy of al dente according to the package directions. Drain, then spread onto a sheet pan until cool enough to handle
Preheat oven to 350°. In a food processor, combine 2 cups (260 g) raw cashews, soaked overnight, drained, and rinsed, 1/2 cup (120 mL) plus 1 Tbsp. nut milk, oat milk, soy milk, or store-bought plant-based milk, 1½ Tbsp. white wine vinegar, 1½ Tbsp. nutritional yeast, 1 garlic clove, peeled, left whole, ½ tsp. onion powder, ¾ tsp. finely grated lemon zest (from ½ lemon), and ¾ tsp. flaky sea salt. Purée, scraping down the sides of the bowl if needed, until completely smooth. Use your hands to squeeze the extra liquid out of scant 1½ cups (190 g) chopped canned or jarred artichoke hearts (packed in oil or water), drained, transfer to food processor, and pulse until incorporated, about 5 pulses. You should have about 3 cups cashew ricotta. Taste and season with more salt if needed.
Pour 1½ cups homemade or store-bought marinara sauce in the bottom of a 13x9" baking dish with a cover. Stuff each shell with about 2 Tbsp. cashew ricotta mixture. (If you have a pastry bag, it can make the job easier. If not, use a small spoon.) Place the stuffed shells on top of the sauce. Pour remaining 1½ cups homemade or store-bought marinara sauce into the baking dish, leaving some of the cashew ricotta mixture exposed.
Cover and bake until hot and bubbling and the pasta is completely cooked, about 30 minutes. Let sit for a few minutes, then top with chopped 3 Tbsp. finely chopped fresh parsley and serve warm.
Do Ahead: Shells can be made 3 days ahead. Store in an airtight container and refrigerate.