1. Four to six hours in advance, prepare the marinade: Put the vinegar, oil, mustard, thyme, bay leaf, and pepper in a bowl. Whisk with a fork until emulsified. Place the steak on a shallow dish, pour half of the marinade over it, then flip the meat and coat it with the remaining marinade. Put the dish in the refrigerator for 4 to 6 hours, flipping the steak 2 times. One hour before cooking the meat, remove it from the refrigerator. Step 2:
2. 45 minutes before serving the dish, peel and slice the onions. Put them in a pan and sweat them, stirring constantly, for 15 minutes over low heat. Then add the cream and curry and simmer for 5 minutes. Step 3:
3. Meanwhile, prepare the barbecue coals, or if you don't have a barbecue, preheat the oven grill. Step 4:
4. Pat dry the ribeye steak and grill it for 12 to 15 minutes on each side, depending on how rare or medium you like the meat, flipping it halfway through and not placing it too close to the heat source. You can add a sprig of rosemary as a garnish before the end of cooking. Step 5:
5. Once the meat is cooked, transfer it to a serving dish, pour the contents of the pan into a gravy boat, and bring it to the table for immediate carving.