In a blender or food processor, combine 1/3 cup white vinegar, 1/3 cup brown sugar, 1 tablespoon honey, 1 teaspoon Kosher salt, 1 teaspoon dry ground mustard and 2 teaspoons white onion. Blend well.
Add the 1 cup olive oil and blend until emulsified.
Stir in the 1 tablespoon poppy seeds by hand and serve.
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Notes
For the salad: baby spinach, strawberries, onions and avocado.