In a small bowl, whisk together the ketchup, vinegar, soy sauce, sugar, cornstarch, and water until the cornstarch is fully dissolved. Set aside.
6 Stir-Fry the Wallaby:
7 Heat a wok or frying pan over high heat until it is very hot.
8 Add about 2-3 tbsp of vegetable oil to the wok. Swirl to coat the bottom.
9 Carefully add the velveted wallaby slices to the hot oil in a single layer, being careful not to overcrowd the wok or pan. You may need to do this in batches.
10 Stir-fry the wallaby quickly for 1-2 minutes per side, or until it is just cooked through and lightly browned. Remove the cooked wallaby from the wok and set aside.
11 Stir-Fry the Vegetables and Aromatics:
12 Add another tablespoon of vegetable oil to the wok if needed. Add the sliced capsicums and onion (and apple if using).
13 Stir-fry for 2-3 minutes until they are slightly softened but still crisp.
14 Add the garlic and grated ginger and stir-fry for another 30 seconds until fragrant.
15 Add the pineapple and sauce, bring it to a simmer, stirring constantly, until the sauce thickens and becomes glossy.
16 Now add the cooked wallaby slices to the wok with the sauce and vegetables.
17 Toss gently and serve over hot steamed rice and greens.