Spray 9x13 casserole dish with cooking spray & set aside.
In a skillet, brown your sausage until thoroughly cooked - remove from heat- set aside.
Preheat oven to 375 degrees.
Cook pasta shells al dente to package directions - drain & set aside.
In a small skillet, heat oil & garlic until garlic begins to brown.
Add in spinach & basil & cook until spinach begins to wilt but is still bright green - remove from heat & set aside.
In a medium bowl, combine ricotta, mozzarella, Parmesan, egg, salt & pepper - mix to combine.
Fold in spinach mixture.
Pour about 1/2 the marinara in the bottom of your casserole dish.
Scoop about 1-2 tbsp of the ricotta mixture into each cooked shell & place on top of the marinara in your dish.
Add in remaining marinara.
Sprinkle with cooked sausage evenly over the top of the shells.
Cover with foil & bake 25-30 minutes.
Remove foil & bake an additional 5-10 minutes
Serve with an additional sprinkle of Parmesan cheese.