Boil pasta shells until just shy of al dente – they should have a bit of a bite left to them.
For the Sauce
Heat oil in a large skillet until just shimmering.
Add the garlic and red pepper flakes and cook until fragrant, about 1 minute.
Add the crushed tomatoes, sugar, and lemon zest.
Simmer 7 minutes.
Stir in fresh herbs.
For the Filling
While the pasta cooks and the sauce simmers, combine all the filling ingredients except the spinach and mix well.
Gently fold in the spinach until evenly dispersed. Extreme mixing will turn the ricotta green.
To Assemble
In a 9×13 casserole dish, spread enough sauce to coat the bottom evenly.
Put about 1 -1 1/2 tablespoons of the ricotta mixture in each shell. Some of the shells will break and won’t be useable, usually I end up with a few leftover.
Place stuffed shells in the pan in a single layer.
Spoon remaining sauce evenly over the top.
Sprinkle with 1 cup mozzarella cheese and 1/4 cup parmesan.
Cover with foil and bake 25-30 minutes, until bubbly.
Remove foil and bake an additional 10 minutes, until cheese is fully melted and bubbly.
