Season with a little salt and pepper, then toss with cornstarch.
Cut the top off the red pepper, then cut pepper in half and remove the white membrane and seeds. Cut the remaining pepper flesh into 1-inch chunks.
Wipe mushrooms clean of any dirt using a damp paper towel, then slice into strips.
Cut onion in half, remove peel, and slice into thin half moons.
Peel and finely chop garlic clove.
Heat 1 tablespoon oil in a 12-inch nonstick saucepan over high heat.
Add chicken and sauté until cooked through and lightly browned, about 9 minutes.
While chicken is cooking, whisk together all ingredients for the sauce.
Remove chicken from pan and set aside.
Add shrimp to skillet—sauté for about 2 minutes, or until done (shrimp will turn whitish pink and opaque). Remove from pan.
Add another tablespoon of oil, then add red pepper, mushrooms, red onion, and garlic. Sauté until onions and mushrooms are softened and beginning to brown, about 7-9 minutes.
Add chicken and shrimp back to pan, along with any juices that may have accumulated underneath them.
Stir in baby spinach, a few handfuls at a time, until spinach is wilted slightly.
Whisk sauce again, then add to skillet. Cook for about 2 minutes, until spinach is fully wilted, and sauce has thickened slightly and coats the stir fry. Serve immediately.
Notes
Storage Tips: Store leftovers in an airtight container in the refrigerator for up to 3 days. You may want to add a splash of water to it when you are reheating it. You can either reheat in the microwave (heat at 80% power, stirring every 30 seconds) or in a nonstick skillet on the stove.Recipe Tips: Adjust your heat as needed. You want a high heat to brown the chicken, and the edges of the vegetables as well, since all that adds to the flavor of your stir fry. But if the veggies or chicken are browning or burning before being fully cooked, turn down the heat. The pan will be quite hot after cooking the chicken and shrimp, so you may want to go down to medium-high heat to cook the vegetables. The great thing about a stir-fry is how little time the cooking process takes. Prep all of your ingredients ahead of time, so that the actual cooking is all that needs to be done when you’re ready to eat.To make this recipe super easy, replace the pepper, mushrooms, and onion with a bag of frozen chopped vegetables. And, replace the homemade stir-fry sauce with a store-bought teriyaki sauce. So fast!