In a large stockpot, bring 2 cups of water to a boil. Add the spinach, continuing to feed more into the pot as the leaves quickly wilt, and cook for 2 to 3 minutes total.. Drain thoroughly in a colander, pressing down on the spinach with a large spoon to extract as much liquid as possible.

When the spinach has cooled, gather it up from the colander, squeeze out any remaining moisture between your hands, transfer to a cutting board, and coarsely chop the spinach with a chef’s knife.

In a large saucepan, melt the butter over medium heat; add the onion and sauté, stirring frequently, until soft and translucent, 5 to 6 minutes. Sprinkle in the flour and whisk for 2 minutes. Then, while still whisking, slowly pour in the warm milk; continue whisking until the sauce thickens. Then stir in the cheese until it is fully incorporated into the sauce. Season to taste with salt and pepper. Thoroughly stir in the chopped spinach. Serve immediately or save for later, storing in the refrigerator and gently reheating on the stovetop before serving.