Heat oil in a large skillet over medium heat.
Add garlic and spinach to skillet and saute until spinach is wilted and garlic is fragrant.
In a large bowl, add Ricotta and egg. Whisk together until combined.
Add spinach mixture, cheeses, and Italian seasoning and stir to combine.
Scoop about 2 Tbs of the cheese mixture and stuff into each shell.
Pour ½ cup spaghetti sauce into the bottom of a 9" X 13" baking dish and spread evenly.
Place stuffed shells in the dish and cover with remaining sauce.
Bake uncovered at 375 degrees for 25 minutes.
Uncover and bake for an additional 10-15.