Preheat oven to 400 F.
Boil shells in salted water according to package directions until they are al dente, drain but leave a little water in the pot to prevent the noodles from sticking together.
In a bowl, add ricotta, lemon zest, lemon juice, garlic, fennel, parmesan cheese and salt then stir together and set aside.
In a sauce pan, add olive oil, butter and garlic and sauté until the garlic is fragrant then add milk and bring to a simmer.
In a small bowl, combine cornstarch and water then stir into the milk mixture.
Add the salt, parmesan cheese and lemon juice as the sauce simmers and thickens.
When sauce has thickened, remove from heat and set aside.
In a baking dish, add some of the sauce on the bottom and then begin filling the shells with the ricotta mixture and laying the shells onto the sauce in the baking dish.
Once all of the filling is used, add the rest of the sauce on top of the shells, followed by mozzarella and a sprinkle of parmesan cheese.
Bake for 20 minutes, serve and enjoy.