Serves 4 Ingredients ½ cup extra virgin olive oil 1 tablespoon unsalted butter 1 medium white onion, thinly sliced into half moons 1 bulb fennel, thinly sliced into half moons 4 cloves garlic, smashed and chopped 1 cup white wine 2 pounds mussels, washed, beards removed Juice of 1 lemon 2 cups fresh parsley leaves,…