Preheat the Blackstone griddle to medium-high heat and drizzle vegetable oil across the cooking surface. The oil should shimmer slightly when ready.
Add the diced onion, zucchini, and mushrooms to the griddle. Sauté for 3–4 minutes, stirring frequently until the vegetables soften and develop light golden edges.
Push vegetables to one side of the griddle. Add chicken pieces to the open area and cook for about 5 minutes, flipping occasionally until the edges are browned and the center is fully cooked.
Add steak pieces next to the chicken and cook for 3–4 minutes, turning frequently until nicely seared but still juicy.
Melt butter on the griddle and add minced garlic. Stir quickly for about 30 seconds until fragrant.
Add cooked rice to the griddle and break up any clumps with a spatula. Toss with soy sauce, sesame oil, garlic powder, salt, and pepper.
Combine the rice, vegetables, chicken, and steak together on the griddle. Stir and cook for another 2 minutes so everything becomes evenly coated and lightly crisped.
Remove from heat, garnish with sliced green onions, and serve hot with yum yum sauce if desired.
