In a bowl, whisk together soy sauce, lemon juice, monk fruit powder, garlic powder, sesame oil, rice vinegar, ginger powder, and pepper until the monk fruit powder dissolves completely. I find whisking for about 30 seconds ensures the sweetener doesn't clump up in the marinade. If using xanthan gum for a thicker sauce, sprinkle it in while whisking continuously to avoid lumps. Pat the chicken thighs dry with paper towels, then place them in a bowl or zip-top bag and pour about three-quarters of the marinade over them, reserving the remaining marinade for finishing the stir fry. Cover and refrigerate for at least 30 minutes, or up to 2 hours for deeper flavor development.

While the chicken marinates, prepare your vegetables for efficient cooking. Trim the zucchini and yellow squash, then slice them into half-moons about 1/4-inch thick. Core the bell peppers, remove seeds, and cut them into 1-inch strips. Dice the onion into bite-sized pieces. I like to keep the vegetables slightly larger than you might expect because they'll shrink as they cook, and this ensures they stay tender-crisp rather than mushy. Arrange all prepped vegetables on a cutting board or in separate piles so they're ready to go when you need them.

Heat 1 tablespoon of olive oil in a large skillet over medium-high heat until shimmering. Remove the chicken from the marinade, allowing excess liquid to drip back into the bowl, then place the thighs in the skillet in a single layer. Don't move them around—let them develop a golden-brown crust for 5-7 minutes on the first side, then flip and cook for another 5-7 minutes until the internal temperature reaches 165°F. Transfer the cooked chicken to a clean plate and set aside.

Add the remaining 1 tablespoon of olive oil to the same skillet over medium-high heat. Add the onion first since it takes longest to cook, stirring occasionally for about 2 minutes. Then add the bell peppers and cook for another 2-3 minutes, followed by the zucchini and squash. Season everything with salt and pepper to taste, and continue cooking for 3-4 minutes until the vegetables are tender but still have a slight bite to them. The skillet should remain hot enough that you hear a sizzle when ingredients hit the pan.

Slice the cooked chicken into bite-sized pieces and return it to the skillet with the vegetables. Pour in the reserved marinade from Step 1, stirring everything together gently. Cook for 2-3 minutes over medium heat, allowing the flavors to meld and the sauce to coat the chicken and vegetables lightly. If the mixture seems dry, you can add a splash of water—I like to have a little sauce to coat everything rather than a dry stir fry. Taste and adjust seasoning with additional salt and pepper if needed before serving.