Make Ahead Tip:

You can prepare the entire dish short of baking it in advance. You can even freeze the unbaked shells and then pop them straight from the freezer into a 350 degree oven. Just add more cooking time (about 40 minutes total) if using frozen stuffed shells.

for the shells:

3 Tbsp extra virgin olive oil
1 small white onion, very finely chopped (about 1/3-1/2 cup)
3 cloves garlic, minced
4 cups baby spinach, washed and spun dry
kosher salt
freshly ground black pepper
1/2 cup low-fat ricotta cheese
1/4 cup finely grated parmesan cheese
1/4 cup shredded Beecher’s Flagship cheese (or other white sharp cheddar)
1 Tbsp basil, finely chopped
1 egg lightly beaten
1/2 lb pasta shells cooked just under al dente*

for the sauce:
1/2 stick unsalted butter
1/4 cup all purpose flour
splash of heavy cream (about 2 Tbsp)
1.5 cups whole milk or half n half
1 tsp kosher salt
freshly ground black pepper
1/2 cup parmesan cheese
1/4 tsp truffle oil