In a medium bowl, whisk together mustard, salt, sugar, and Strawberry Vinegar. While vigorously whisking, pour 2 to 3 tablespoons oil into mix and whisk until well incorporated. In a thin, steady stream, whisk in remaining oil. Add spinach and strawberries; gently toss well to coat. Sprinkle poppy seeds over the top.
Place strawberries and vinegar in a jar with a tight-fitting lid. Let stand in a sunny window 3 to 4 days (the vinegar will become bright red). Strain into a small saucepan and discard berries. Stir in sugar and heat over medium heat until mixture comes to a boil. Simmer over low heat about 10 minutes. Cool and pour into a clean bottle. Cover and store in a cool, dry place or in the refrigerator. Yield: about 2 cups.