Chimichurri Flank Steak with Grilled Radicchio and Onions

Ingredients

• ½ teaspoon coarse salt
• ¼ teaspoon fresh ground black pepper
• ¼ cup sherry wine vinegar
• 2 tablespoons coarsely chopped basil leaves
• ¼ cup oregano leaves
• ½ cup coarsely chopped parsley
• ½ cup coarsely chopped mint leaves
• 4 cloves garlic cloves, peeled and minced
• 2 medium shallots, finely minced
• kosher salt
• freshly ground pepper
• 2 teaspoons crushed red pepper (to taste)
• 1½ pound flank steak about 1 inch thick
• 1 large head of radicchio, well rinsed, outer leaves removed
• 1 large yellow onion, sliced into ½ inch intact discs
• extra-virgin olive oil
• sea salt or kosher salt and freshly ground black pepper
• more sherry wine vinegar, optional