Step 1 Drain 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 2 Remove 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.