Grilled London Broil with Summer Sauce

Name: Grilled London Broil with Summer Sauce
Ingredients: 2.5 pounds London Broil beef (boneless top sirloin) at least 1-1/4 inch thick; 2 tablespoons cracked green peppercorns; 2 tablespoons cracked black peppercorns; 1 tablespoon soy sauce or tamari; 1 tablespoon butter; 1 tablespoon olive oil; 1 shallot, minced; 2 garlic cloves, minced; 8 ounces cremini mushrooms, sliced thinly; 1 vine-ripened tomato, chopped; 1 tablespoon capers; 18 ounces canned artichoke hearts, drained; 1/2 cup kalamata olives, whole, pitted; 1/2 cup dry sherry wine; 1 teaspoon balsamic vinegar; salt and pepper; fresh chopped tarragon, flat-leaf parsley, and chives