Make the marinade: in a mixing bowl whisk together chopped lemongrass, vegetable oil, fish sauce, light brown sugar, lime juice, soy sauce, and minced garlic until smooth; alternatively blend on high for 30 seconds.
Marinate the chicken: place the chicken strips and the marinade into a resealable plastic bag, seal, and refrigerate for 1 to 2 hours, turning the bag occasionally to coat evenly.
Thread chicken onto soaked bamboo skewers, distributing pieces evenly.
Heat the grill: preheat the grill with the lid closed on high for 10 minutes, then open and set to medium-high heat.
Grill the skewers: place skewers on the grill and cook until browned and cooked through, about 3 minutes per side (adjust time as needed for thickness).
Make the mango tahini sauce: combine peeled, pitted mangoes, tahini, 1 garlic clove, toasted sesame oil, rice vinegar, and grated ginger in a blender or food processor; process until smooth.
Serve: plate the grilled chicken kabobs with mango tahini sauce and optional fresh mango spears or garnishes as desired.