Step 1Combine dry mustard, water, oil, soy sauce, vinegar, Sherry, salt, ginger, garlic and sugar in bowl. Set aside. Makes 1/2 cup.
Step 1Fill wide skillet with water and bring to boil. Wash spinach well, discarding stems and any yellow or bruised leaves. Reserve 8 of the crowns (the stems that are joined at the roots) and wash them too. Cut tofu into small cubes.
Step 2When water boils, add 1 teaspoon salt and spinach. Cook just until wilted, turning it a few times with tongs, then lift spinach into strainer and rinse under cool water. Set aside. Add crowns to pan, cook about 15 seconds, then remove and rinse. Turn off heat and slide tofu into water. Let sit 1 minute, then remove onto paper towels to dry.
Step 3Roast sesame seeds in small skillet over medium heat until fragrant, 1 to 2 minutes.
Step 4Squeeze water out of spinach and chop, neither too fine nor too coarsely. Set in bowl and toss with 3 tablespoons dressing and about two-thirds sesame seeds. Taste and add more dressing, if desired. Distribute spinach among 4 small plates, then place tofu around and on top. Spoon more dressing over tofu, then garnish with slivered, pickled ginger and remaining sesame seeds. Place 2 spinach crowns on each plate and serve.