Fill a large Dutch oven or pot 1/2 with water, bring to a boil.
Add green beans. Boil for 2 minutes. Drain and transfer to a mixing bowl fitted with a colander and filled with ice water. When cooled, drain in the fitted colander and transfer to a mixing bowl.
Place blanched green beans, butter beans and wax beans in a large mixing bowl.
In a separate small bowl, whisk together Dijon mustard through black pepper.
Toss with beans. Refrigerate and cover for 4-24 hours.
Before serving, allow to sit at room temperature for 20-30 minutes and toss to recoat. You might need to drain some of the liquid before serving.
If you've tried this recipe, come back and let us know how it was!
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Notes
Prepared green beans just means that the woody ends are snipped with kitchen shears or snapped off.