Heat a frying pan with oil and butter over medium heat. Add the onion & sauté for five or so minutes, stirring frequently. Add the garlic and spices, and cook, stirring occasionally, for 1-2 minutes. Add the kale and silverbeet, and cook, stirring occasionally, for 6-8 minutes until wilted. Reduce heat to low and cook, stirring occasionally, until only a little bit of moisture remains. Season to taste.