Salisbury Steak with Mushroom Sauce

Name: Salisbury Steak with Mushroom Sauce
Ingredients: 1 onion, half minced, half sliced into rings; 1½ tablespoons olive or vegetable oil, divided; 1½ pounds ground beef (85/15, you want these juicy); ¼ cup breadcrumbs or crushed crackers; 1 egg, slightly beaten; 1 tablespoon horseradish; 1 tablespoon Worcestershire sauce, divided; Salt and pepper to taste; 3 cups mushrooms, sliced; 1 14.5-ounce can beef broth, low sodium; 1 tablespoon cold water; 2 teaspoons cornstarch