Heat butter in a 6-qt. pot over medium heat. Add cardamom, fenugreek, and nigella and cook, stirring often, until fragrant, 1–2 minutes. Increase heat to medium-high and add oil; add onions and cook, stirring often, until browned, 10 minutes. Add garlic, chiles, and ginger and cook, stirring often, until soft and fragrant, 3 minutes. Add collards, 1⅓ cups water, and salt and pepper; cover and bring to a boil. Reduce heat to low and cook, stirring occasionally, until collards are tender, 50–55 minutes. Stir in vinegar and serve collards hot.