Start the oven: Heat the oven to 400°F. Set a large pot of water on the stove over high heat so it can come to a boil for the shells.

Lightly oil a 9-by-13-inch baking dish. Spoon about 1 cup of the marinara sauce into the dish and spread it into an even layer.

Boil the shells: When the water boils, add a generous pinch of salt. Put the jumbo shells into the pot.

Cook the shells for 2 minutes less than the package says. They should be flexible enough to fill but still a little firm in the center.

Drain the shells in a colander. Rinse them gently under cool water so they stop cooking, then set them on a sheet pan or large plate so they do not stick together.

Warm the sauce: Set a large skillet over medium heat and add the olive oil.

Add the chopped onion to the skillet. Cook for 5 to 6 minutes, stirring now and then, until the onion is soft and lightly golden.

Add the minced garlic and dried oregano to the skillet. Stir for 30 seconds, just until the garlic smells fragrant.

Pour the remaining marinara sauce into the skillet with the onion mixture. Stir and let the sauce simmer over medium-low heat for 5 minutes.

Make the filling: Put the drained butter beans into a medium bowl. Mash them with a fork until they are broken up and mostly chunky-smooth.

Add the ricotta, the squeezed-dry spinach, 1/4 cup of the Parmesan, 1 teaspoon of the kosher salt, and the black pepper to the bowl. Stir until the filling is evenly mixed.

Taste the filling and add a little more salt if it needs it. The filling should taste well seasoned because the shells and sauce are mild.