Elsie Maxwell always has a carafe of strawberry vinegar on her counter, ready for action.
In a large bowl combine spinach and strawberries.
In a smaller bowl, whisk together Strawberry Vinegar, sugar, mustard, and salt. While still whisking, drizzle oil into mixture to make a vinaigrette.
Add mixture to spinach and strawberries and toss to coat. Sprinkle with poppy seeds.
Place strawberries and vinegar in a jar with a tight lid. Let stand in a sunny window 3-4 days (the vinegar should be bright red).
Strain into a small saucepan and discard berries. Stir in sugar; heat on medium setting 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.