Heat oven to 350° and heat a skillet on medium high heat with olive oil. Once hot, add minced onion and cook until translucent. Stir in finely minced garlic until fragrant, 1-2 minutes, then remove from the heat.
Grab a medium bowl and either shred the chicken into very small pieces, or mince it finely *see note 5. Squeeze all of the moisture from the spinach *see note 6, then add it to the bowl along with the crumbled feta, egg, lemon juice, fried onion and garlic, salt and pepper. Mix well.
Fill cannelloni noodles until all of the chicken mixture is used up; you should get about 10 cannelloni noodles filled.
Spread 1/2 cup marinara all over the bottom of a 9x13" baking dish. Space the chicken cannelloni around the baking dish, being sure to leave space in between because the noodles will spread while they bake.
Pour 1 1/2 cups marinara evenly over the noodles; be sure some of the sauce goes in between each noodle. The noodles do not need to be submerged in the sauce, there just needs to be a little bit over each one.
Pour the heavy cream over the middle of the noodles and top with the grated parmesan cheese. Cover with tin foil and bake for 45 minutes. Uncover the dish and then bake for an additional 15 minutes *see note 7. Serve hot and and enjoy!