Prepare rice according to package directions and keep warm.
Combine cornstarch, fish sauce, the water, and one minced garlic clove in a shallow bowl. Toss mixture with chicken tenders, coating evenly. Heat 2 teaspoons oil in a large nonstick skillet on medium-high. Add chicken and cook 3 minutes per side. Remove chicken from pan. Heat remaining oil in pan and sauté green onions for a minute. Add the remaining garlic and the ginger and cook another minute.
Return chicken to pan for 1 to 2 minutes, or until it's cooked. Stir in coconut milk, lime juice, sugar, and, if you're using it, green sauce. Cook about a minute until heated through. Serve over brown rice with chopped cilantro, if using.