1/4 cup olive oil (divided)
1 medium yellow onion (halved lengthwise, then thinly sliced crosswise, about 1-1/2 cups)
2 large garlic cloves (finely minced)
1 pound kale (center ribs removed, leaves chopped, about 10 cups firmly packed)
1/3 cup chicken or vegetable broth (or water)
1 cup panko (Japanese-style) bread crumbs
1 tablespoon minced fresh tarragon
1 tablespoon white or black truffle oil
  Salt and freshly ground black pepper
1 1/2 cups (3 ounces) grated Gruyère or Comté cheese