In a large skillet over medium-high heat, add ground turkey along with olive oil. Cook the turkey, breaking it up with a spatula as it cooks until it is cooked through to 165˚F, about 6-7 minutes.
Add diced onion, garlic, salt, pepper, and sage. Cook for another 3-4 minutes until veggies soften. Remove from heat and let cool.
When the turkey mixture has cooled, add it to a medium bowl with ricotta cheese and fresh basil. Mix together well.
Cook manicotti shells according to package. Drain and rinse well to stop the cooking.
Working with one shell a time, stuff the turkey filling in the manicotti shell. You should be able to use all the filling for the manicotti. Stuff them full! You should be able to stuff at least 12 manicotti shells and can maybe get 14.
Transfer stuffed manicotti to an 8x11 baking dish with 1½ cups of marinara sauce in the bottom. Drizzle each manicotti with a little extra sauce and sprinkle with mozzarella cheese.
Bake the manicotti, covered with foil, in a preheated 350˚F oven for 20 minutes.
Then remove the foil and bake for another 10-15 minutes until the cheese is bubbling and melted. Garnish baked manicotti with fresh parsley and serve while warm.