Heat the oven to 425°F with a rack in the middle. Set out a 10-inch ovenproof skillet or a 2-quart baking dish.

Start the sauce: Put the ovenproof skillet on the stove over medium heat and add the olive oil.

Add the chopped onion to the skillet with 1/2 teaspoon of the kosher salt. Cook for 5 to 6 minutes, stirring often, until the onion is soft and lightly golden at the edges.

Add the sliced garlic to the skillet. Cook for 30 seconds, stirring, just until it smells fragrant.

Add the tomato paste, dried oregano, and 1/4 teaspoon of the black pepper to the skillet. Stir for 1 minute until the tomato paste looks a shade darker and coats the onion.

Pour the crushed tomatoes and 1/2 cup water into the skillet. Stir well and scrape the bottom of the pan so nothing sticks.

Bring the sauce to a gentle simmer. Let it cook for 4 to 5 minutes, stirring once or twice, until it thickens slightly.

Add the drained butter beans to the skillet. Stir gently so the beans stay mostly whole.

Simmer the beans in the sauce for 3 to 4 minutes, until they are heated through and the sauce is bubbling in small, steady bubbles.

Add the spinach to the skillet a few handfuls at a time. Stir after each addition until the leaves wilt down and make room for the rest.

Taste the sauce and add more salt and black pepper if needed. The sauce should taste well seasoned now because the beans and feta will soften the flavor a little.

Build the casserole: If your skillet is not ovenproof, transfer the bean mixture to the baking dish. Spread it into an even layer.