Cook the pasta. Bring a large pot of salted water to a boil. Add the pasta and cook until al dente according to the package directions. Before draining, reserve about 1 cup of the starchy pasta water. Drain the pasta and set aside.
Cook the onion. While the pasta cooks, heat the olive oil or reserved sun-dried tomato oil in a large 12-inch deep skillet or Dutch oven over medium heat. Add the diced onion and cook for 3–4 minutes, stirring occasionally, until softened and translucent.
Add the garlic. Stir in the minced garlic and cook for about 30–60 seconds, stirring constantly, just until fragrant. Don’t let the garlic brown.
Blend the sauce. Transfer the cooked onion and garlic to a blender. Add the sun-dried tomatoes, non-dairy milk, maple syrup, lemon juice, tahini, basil, salt, black pepper, and red pepper flakes. Blend until smooth and creamy. The sauce should be pourable rather than thick like a dip.
Combine the pasta and sauce. Pour the blended sauce back into the skillet and warm it gently over medium-low heat. Add the drained pasta and toss until evenly coated. Add the reserved pasta water a few tablespoons at a time as needed, until the sauce is silky and easily coats the pasta.
Add the spinach. Add the spinach and toss over low heat just until wilted. Taste and adjust the salt, black pepper, and red pepper flakes as needed.
Serve. Remove the skillet from the heat while the sauce is still slightly loose, since it will continue to thicken as it sits. Serve warm with extra fresh basil and vegan Parmesan, if desired.
