Preheat the oven to 350°F (or 175°C). Lightly grease a 9×13-inch baking dish with olive oil.
Cook the pasta shells. Bring a large saucepan of water to a boil and generously salt the water. Add the jumbo pasta shells and cook according to the box instructions until just al dente (do not overcook!), then drain well.
Transfer the cooked pasta shells to a baking sheet and drizzle with the 1 tablespoon olive oil to prevent sticking and set aside.
While the pasta is cooking, prepare the Tofu Ricotta. Drain the block of tofu and dab away the excess water with some paper towels. Place the tofu, nutritional yeast, garlic powder, onion powder, salt, pepper, extra virgin olive oil, lemon zest, lemon juice, and crushed red pepper flakes (if using) in a food processor and pulse repeatedly until the texture is smooth but still a little chunky, similar to the texture of ricotta cheese.
Make the pumpkin ricotta filling. In a large bowl, combine the Tofu Ricotta, pumpkin purée, nutmeg, 1/2 teaspoon kosher salt, black pepper to taste, and vegan parmesan cheese (if using). Fold the ingredients together using a silicone spatula. Taste the filling and adjust the seasonings accordingly, adding more salt, pepper, or nutmeg as needed. Spoon 1-2 tablespoons of the filling into each jumbo pasta shell, and set aside.
Make the Garlic Béchamel Sauce. Heat a large frying pan over medium heat and add the 3 tablespoons extra virgin olive oil. Once the oil is hot, add onion and season with a pinch of salt. Cook for 6-7 minutes, or until the onion is softened and lightly browned. Add the garlic and cook for 1 to 2 minutes, stirring frequently to prevent burning. Reduce the heat to medium-low and add the flour, whisking constantly for a minute until well incorporated. Gradually pour in the lite coconut milk in stages, whisking after each addition to prevent clumps, and then cook for 2 minutes until the sauce has thickened.
Transfer the sauce to a blender and add the 1 teaspoon kosher salt and black pepper to taste. Blend until the sauce is smooth and creamy. Taste the sauce for seasonings, adding more salt or pepper as desired.
Assemble the dish. Pour 1/2 cup of the Garlic Béchamel Sauce in the prepared baking dish. Arrange the stuffed jumbo shells on top of the sauce. Pour the remaining sauce on top of the shells. Scatter the chopped fresh sage leaves on top of the sauce. Bake the stuffed shells for 25 minutes, or until the pasta shells are lightly golden. Serve warm.