In a medium-sized bowl, combine the finely chopped lemongrass, minced garlic, fish sauce, soy sauce, brown sugar, turmeric powder, lime juice, chili flakes (if using), and vegetable oil. Stir everything together until the sugar is dissolved and the ingredients are evenly mixed.

Place the chicken thighs in a resealable plastic bag or shallow dish. Pour the marinade over the chicken, ensuring the pieces are well-coated. Seal the bag or cover the dish with plastic wrap. Refrigerate for at least 1 hour, or overnight for the best flavor infusion.

While the chicken marinates, thinly slice the onion and julienne the red bell pepper. These vegetables will be sautéed alongside the chicken for added texture and flavor.

Chop fresh cilantro and prepare lime wedges for garnish and serving.

In a large skillet or wok, heat 2 tablespoons of vegetable oil over medium-high heat. Once the oil is hot, add the onion and bell pepper slices. Sauté for about 3 minutes, or until they become tender and slightly caramelized. Set them aside for later use.

In the same skillet or wok, add a bit more oil if needed. Place the marinated chicken thighs in the pan and cook them over medium-high heat for about 4-5 minutes per side, or until the chicken is golden brown and cooked through. The internal temperature should reach 165°F (74°C).

Once the chicken is cooked, remove it from the pan and let it rest for 5 minutes. This resting period allows the juices to redistribute, keeping the meat moist and flavorful.

After the chicken has rested, slice it into strips or bite-sized pieces. Return the sautéed onions and peppers to the pan, and toss them together with the chicken. Optionally, drizzle a little sesame oil over the mixture for a deeper flavor.

Plate the chicken and vegetable mixture, and garnish with fresh cilantro. Serve with a side of steamed jasmine rice, lime wedges, and any additional sides such as fresh vegetables or a simple salad for added crunch.