Heat the oven to 425°F. Lightly oil a 2-quart baking dish or 9-inch square baking dish and set it near the stove.

Fill a large pot with water and set it over high heat so it can come to a boil for the tortellini.

Set a large skillet 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 a few times, until the onion is soft and lightly golden at the edges.

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

Start the sauce: Pour the marinara sauce and the 1/2 cup water into the skillet. Stir well and bring the sauce to a gentle simmer.

Add the drained cannellini beans to the skillet. Simmer for 4 to 5 minutes, stirring once or twice, until the beans are hot and the sauce looks slightly thickened.

Add the baby spinach to the skillet a handful at a time. Stir after each handful until the leaves wilt down into the sauce.

Season the sauce in the skillet with the black pepper. Taste and add a little more salt if needed, then turn off the heat.

Cook the tortellini: When the water in the pot reaches a boil, add 1 teaspoon of the kosher salt.

Add the cheese tortellini to the boiling water. Cook it for 2 minutes less than the package directs, until the pasta is puffed and flexible but still a little firm in the center.

Drain the tortellini in a colander. Do not rinse it.