Ingredients
- 3 tablespoons unsalted butter , softened, divided
- 1 1/2 tablespoons flour
- 1/4 cup finely chopped onion
- 25 to 30 ounces fresh baby spinach leaves
- 1/2 teaspoon kosher salt
- 1/8 teaspoon freshly ground black pepper
- 2 tablespoons water
- 1/2 cup heavy cream
- 1 1/2 teaspoons D ijon mustard , or to taste
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