Put the thawed spinach in a clean kitchen towel or several layers of paper towels. Squeeze over the sink until very little liquid comes out; dry spinach helps the fritters brown instead of steam.

Put the cannellini beans in a large bowl. Mash them with a fork or potato masher until they are mostly broken down but still a little chunky.

Add the squeezed spinach to the bowl with the beans.

Add the grated onion to the bowl.

Pour the beaten egg into the bowl.

Add the flour, 1/4 cup of the Parmesan, the salt, and the black pepper to the bowl. Stir until the mixture is thick and holds together when pressed with a spoon.

Let the bowl stand for 5 minutes so the flour can absorb some moisture and firm up the mixture.

Pour the marinara and water into a medium saucepan. Put the saucepan over low heat and warm the sauce until it starts to bubble gently, about 5 minutes.

Line a plate with paper towels. Put a large nonstick skillet over medium heat and add 2 tablespoons of the olive oil.

Scoop about 1/3 cup of the fritter mixture into the skillet for each fritter. Flatten each mound into a patty about 1/2 inch thick with the back of a spoon, leaving space between them.

Cook the fritters until the first side is deep golden brown and the edges look set, 3 to 4 minutes.

Turn the fritters with a thin spatula. Cook the second side until deep golden brown and firm, 3 to 4 minutes more.