In a mixing bowl, whisk together the chicken broth, soy sauce, seasoned rice vinegar, minced garlic, and grated ginger.
Add the cubed chicken thighs to the marinade, ensuring each piece is well-coated. Let it marinate for at least 15 minutes.
In a separate bowl, toss the marinated chicken pieces with 1/4 cup of cornstarch until evenly coated.
In a large skillet or wok, heat 2 tablespoons of sesame oil over medium-high heat. Add the coated chicken pieces and cook for about 5-7 minutes per side until golden brown.
Remove the chicken from the skillet and set aside. In the same skillet, add the remaining 2 tablespoons of sesame oil and sauté the sliced mushrooms and zucchini for about 4-5 minutes.
Mix 3 tablespoons of cornstarch with 1/4 cup of water to create a slurry. Pour the marinade into the skillet, bring it to a simmer, and stir in the cornstarch slurry to thicken the sauce.
Return the cooked chicken to the skillet, tossing to coat it well with the sauce and vegetables. Cook for another minute to heat everything through.
Serve your homemade Panda Express Mushroom Chicken hot with steamed rice or noodles.