In a mixing bowl, whisk together one can of coconut milk, two tablespoons of red curry paste, and two teaspoons of fish sauce (if using). Add a handful of chopped cilantro, the juice of one lime, one teaspoon of kosher salt, and two tablespoons of brown sugar. Stir until smooth and well combined.

Place the boneless, skinless chicken thighs in a shallow dish or resealable plastic bag. Pour the marinade over the chicken and ensure all pieces are evenly coated. Seal or cover and let marinate for at least 30 minutes.

Preheat your grill on high for about 10 minutes, then reduce the temperature to medium-low, aiming for around 350°F to 375°F. Lightly oil the grill grates.

Remove the chicken from the marinade, shaking off any excess, and place it on the preheated grill. Cook for about 4 to 5 minutes on one side until grill marks appear. Flip the chicken, brush with reserved marinade, and cook for another 4 to 5 minutes until it reaches an internal temperature of 165°F.

In a large bowl, combine sliced Persian cucumbers, chopped green onions, and more cilantro. In a small bowl, whisk together the juice of one lime, one tablespoon of toasted sesame oil, one teaspoon of fish sauce (if using), two teaspoons of brown sugar, and a pinch of crushed red pepper flakes. Mix well and gently smash the cucumbers.

Once the chicken is grilled, transfer it to a platter. Top with cucumber salad and garnish with extra cilantro if desired. Serve immediately.