Sweet and Sour Lamb Stew



1 ½ pound boneless leg of lamb (cut into bite size chuncks)
2 tablespoon all-purpose flour
½ teaspoon kosher salt (plus more for seasoning)
¼ teaspoon freshly cracked black pepper (plus more for seasoning)
¼ cup olive oil
1 medium Maui or Vidalia sweet onion (peeled, halved lengthwise then sliced crosswise into thin strips)
2 cup chicken broth (plus up to 2 cups more more as needed)
⅓ cup apple cider vinegar (or as much as ½ cup if you like the sharpness)
2 tablespoon tomato paste
2 tablespoon brown sugar
1 cup dried currants
1 ½ ounce toasted pine nuts (about ½ cup)
mint sprigs (as garnish, optional)