Preheat the oven to 375°F. Lightly oil shallow a 2-quart casserole, or spray with cooking spray.
Bring a large pot of water to a boil, and salt it generously. Cook the pasta according to package directions, stopping just before the pasta is fully tender. Right before draining the pasta, scoop out 2 cups of the cooking water and reserve, then drain the pasta and return the shells to the pot.
While the pasta is cooking, heat 2 tablespoons olive oil in a large skillet over medium-high heat. Add the onion, season with salt and pepper and sauté the onion for 5 minutes until lightly browned. Stir in the garlic and sauté for one more minute, until you can smell the garlic. Stir in the spinach, stirring just until it is slightly wilted, about 2 minutes. Add the lemon zest, goat cheese, pureed roasted pepper (if using) and the reserved 2 cups cooking water, then stir to combine well, until the goat cheese is dissolved into a creamy sauce. Turn the mixture into the pot with the cooked pasta and stir to blend well.
Turn the mixture into the prepared casserole pan and sprinkle the top with the sunflower seeds. Bake for 20 to 25 minutes until it is bubbly around the edges and the top and the sunflower seeds have turned golden brown. Serve hot.