Step 1Drain the tofu on paper towels for about 15 minutes. Pat the top and sides with a paper towel to remove any excess moisture. Cut the tofu into 1/2-inch cubes and let them stand on the paper towels until you're ready to add them to the spinach.
Step 2Remove the stems from the spinach. Rinse the spinach leaves and drain them on paper towels. Pat them dry and coarsely chop the leaves.
Step 3Heat the butter in a large 5-quart saucepan until melted. Add the onion and saute it over medium heat about 5 minutes. Stir in the ginger and continue to saute until the onion is tender but not browned, 2 to 3 more minutes. Add the spinach to the pan, cover it and cook until the spinach has totally wilted, 2 to 3 minutes, stirring occasionally.
Step 4Stir in the salt and flour. Stir in the milk. Heat and stir until the sauce thickens, about 1 minute. Using a hand blender, puree until the mixture is almost smooth. Stir in the cubed tofu. Heat until hot, then serve.