Preheat the oven to 375ºF.
If your pasta box instructs you to, boil the pasta in salted water for as long as it states. Some brands may claim to be "oven ready", in which case you don't have to boil them, but you may choose to boil for 2 minutes just to make them slightly pliable. Drain and rinse the pasta with cold water to stop cooking.
Simultaneously, add the olive oil to a pan over medium heat. Sauté the onion for 3 minutes before adding the garlic and spinach. Cook until the spinach is wilted and the onion is softened. Remove to a medium size bowl and let cool for 5 minutes.
To the sauté mix, add to the ricotta, egg, parmesan, zest of the lemon, torn basil leaves, Italian seasoning, chili flakes, and salt. Mix with a spatula until combined. Transfer the filling mix to a piping bag or a ziplock bag and cut the end.
Spread half a jar of tomato sauce across the bottom of a 9x13" baking dish.
Pipe the filling all the way through each gluten free cannelloni shell. Place the filled tube in the baking dish on top of the tomato sauce. Repeat until all cannelloni are filled.
Top the cannelloni with more tomato sauce, then a layer of mozzarella cheese. Cover with tin foil and bake for 20 minutes covered, then 10 minutes uncovered. Broil on high for 3-5 minutes at the end to brown the cheese.
Remove from the oven and serve immediately. Sprinkle with parsley, fresh basil leaves, and more red chili flakes if you like. Be careful, the pasta will be hot!