Rub the steaks with fennel and coriander seeds then season generously with salt and pepper. Drizzle with half the olive oil, then set aside.
Fry the onions and chilli in the remaining olive oil for 5 minutes until lightly browned, then increase the heat and add the butter. Cook until caramelised. Season with salt then mix in the vinegar and sugar.
Cook the steaks over a hot barbecue grill for 10 minutes, turning regularly, until medium-rare. Rest briefly. Spread onions on plates, then top with steaks, tomatoes, basil and burrata.