Start by preheating your oven to 425°F so it’s ready when you are.
Steam the spinach by placing it in a steamer basket over simmering water. Cover and cook for about 1 minute, just until wilted. Transfer it to a strainer, squeeze out all the extra water, and give it a rough chop.
Meanwhile, bring a large pot of well-salted water to a boil. Cook the jumbo shells for about 10 minutes, until al dente. Drain and drizzle with a little olive oil so they don’t stick together.
In a medium bowl, mix the chopped spinach with ricotta, pecorino, garlic, oregano, lemon zest, red pepper flakes, salt, and a few grinds of black pepper until well combined.
Spread marinara sauce across the bottom of a 9×13 baking dish. Stuff each shell generously with the ricotta mixture and nestle them into the sauce.
Cover the dish with foil and bake for 20 minutes, until everything is warm and bubbly. Serve with extra marinara and finish with fresh parsley and more cheese if you like.