Trim excess fat off the chicken then, cover with a sheet of parchment paper or plastic wrap and using a meat mallet or the bottom of a heavy skillet, pound the chicken until it is a uniform ¼ inch thickness. Place the tenderized chicken in a shallow bowl and add 1 tbsp of the olive oil, soy sauce, lemon juice, salt and pepper and toss until well combined. Set aside to marinade at room temperature for 15 minutes or, alternatively, you can cover and refrigerate overnight to marinade.
Heat a large skillet with tall-sides over medium-high heat with the remaining 1 tbsp olive oil. When hot, add the chicken and cook until golden brown on each side and just cooked through, 3 to 4 minutes per side. Transfer to a plate and set aside to let rest while you prepare the veggies.
Add more olive oil (if the skillet looks dry and it is needed) to the skillet along with the bell pepper, onion, mushrooms, tomatoes, garlic, 1 tbsp of soy sauce, and a pinch of salt and pepper. Cook, stirring occasionally, until the bell peppers are slightly tender, 3 to 4 minutes. Add the spinach and continue to cook, stirring until just wilted, about 1 minute. Remove from heat.
To serve, divide the rice amongst 4 bowls, then divide up the sauteed veggies. Dice up the cubed chicken and divide amongst the bowls. Serve with sriracha, if desired, and enjoy!