Cook meat and drain. Set aside.
Set your slow cooker to low.
Add one jar of marinara sauce to the bottom of your slow cooker.
Arrange all the shells, opening faxing up at the bottom of your slow cooker.
In a mixing bowl, mix together ricotta cheese, meat, the egg, and any other preferred fillings
Using a small cookie scoop, or spoon, fill each shell with the ricotta-meat mixture.
Pour the second jar of marinara sauce over the stuffed shells.
Cook on low for 4-6 hours.
Sprinkle mozzarella cheese over the stuffed shells a few minutes before serving.