In a small bowl, whisk the Hong Kong sauce with the lemon juice and zest. Set aside. \nCoat a large skillet or wok with a drizzle of canola oil and heat over medium heat. Add the onion and cook until soft and translucent, about 5 minutes. Add in the apple, shrimp, and half of the Hong Kong sauce mixture. Continue cooking until the shrimp have turned pink and apples have softened, about 5 minutes more. \nTurn heat to low and add the kale, remaining Hong Kong sauce mixture and a pinch each of kosher salt and Korean chili flakes, if desired. Stir until the kale has wilted and all ingredients are coated and combined, about 3-5 minutes more. Season to taste with salt. Serve as is, or over cooked rice.