Heat 2 tbsp olive oil in a medium skillet over medium heat. Add the onion and half the garlic and cook, stirring occasionally, until soft, about 4 minutes. Add the beans with their liquid, the salt and 1/4 cup water and bring to a simmer. Cook until the beans are warmed through, about 3 minutes. Scrape everything into a food processor, add 2 tbsp olive oil, the preserved lemon and cumin and blend until smooth. Heat the remaining 1 tbsp olive oil in a medium skillet over medium heat. Add the Swiss chard and the remaining garlic and cook, stirring occasionally, until soft, about 4 minutes. Spread the bean dip in a shallow bowl and pile the greens on top.