Rinse the tuna, pat dry, season with salt and pepper.
Heat the oil in a large saucepan and fry the onions in it until translucent, push aside and fry the fish slices briefly. Add the tomatoes and let simmer for 10 minutes. Pour the cognac over the fish, ignite and let it burn out.
Add the wine with the rinsed, peeled thyme leaves and the bay leaf and cover the tuna for 10 minutes, then simmer openly for another 10-12 minutes.
Remove the bay leaf. Arrange the fish on a preheated platter. Season the stewed stock with salt and pepper, spread over the pieces of fish and sprinkle generously with parsley and garlic.