Start by slicing the sirloin steak into thin strips against the grain. Wash and slice the yellow squash, onion, red bell pepper, and shiitake mushrooms. Mince the ginger and garlic, setting everything aside.
In a bowl, combine the sliced beef with 1 tablespoon of sesame oil, kosher salt, and black pepper. Marinate for about 10 minutes.
Heat 1 tablespoon of sesame oil in a large skillet or wok over medium-high heat. Add the marinated beef and stir-fry for 3-4 minutes until browned and cooked through. Remove beef and set aside.
In the same skillet, add the yellow onion, shiitake mushrooms, yellow squash, and red bell pepper. Stir-fry for 4-5 minutes until tender yet crisp. Add minced ginger and garlic; cook for 1 more minute until fragrant.
Return the beef to the skillet with vegetables. Mix BBQ sauce, honey, and soy sauce in a small bowl, then pour over beef and vegetables. Stir and cook for 1-2 minutes until coated and heated through.
Remove from heat and serve immediately. Enjoy with steamed rice, noodles, or on its own.