Steak and Vegetables with Yum Yum Sauce

Name: Steak and Vegetables with Yum Yum Sauce
Ingredients: 1 1/2 lb. boneless sirloin or strip steak; 1 1/2 tsp. kosher salt; 1/2 tsp. black pepper; 4 tbsp. salted butter, at room temperature; 4 garlic cloves, finely chopped; 2 tbsp. vegetable oil; 2 tbsp. mirin or dry sherry; 1/4 c. low-sodium soy sauce; 2 medium zucchini, cut into 2-inch sticks; 1 sweet onion, cut into 1-inch pieces; 1 lb. cremini mushrooms, quartered; Steamed white rice, for serving; bottled yum yum sauce, for serving