Butter beans, or small lima beans, are fresh in the summer and can be frozen to use year-round. Fresh butter beans should be cooked for 25 to 30 minutes much longer than frozen lima beans. Any extra vinaigrette can be stored in an airtight container and refrigerated. 

Ingredients
2  pounds fresh green beans, ends snapped    
  Salt    
1  pound 4 ounces butter beans, or small fresh or frozen lima beans    
1/4  cup tarragon or white-wine vinegar    
3  tablespoons Dijon mustard    
  Freshly ground black pepper    
1  shallot, minced    
3/4  cup extra-virgin olive oil    
2  tablespoons chopped fresh tarragon, plus more for garnish    
2  tablespoons chopped fresh flat-leaf parsley, plus more for garnish  

Directions  
  1.   Prepare an ice-water bath. Cook green beans in boiling salted water, just until bright green and tender, about 5 minutes. Reserving the cooking liquid, remove beans with a large slotted spoon; transfer to ice-water bath to stop the cooking. Transfer beans to a colander. Cook butter beans in the boiling cooking liquid, just until tender, 5 minutes if frozen. Strain; place under cold, running water to stop cooking. Set aside.
    
  2.   Make vinaigrette: In a medium bowl, combine vinegar, mustard, salt and pepper to taste, and minced shallot. Whisk in olive oil until vinaigrette is creamy. Whisk in tarragon and parsley. 
   
  3.   In a large bowl, toss green beans until well coated with about three-quarters of the vinaigrette. In a large bowl or on a platter, arrange the green beans in an even layer. Sprinkle the reserved butter beans over the top. Drizzle the remaining vinaigrette over the beans. Garnish with tarragon and parsley. Serve at room temperature.  Serves 10.

Source:  www.marthastewart.com 
Photo by: Chris Baker