To marinate the steaks, pour the wine into a non-metallic dish. Cut the garlic bulb in half horizontally and add to the wine.
Coarsely crush the peppercorns in a pestle and mortar and add the rosemary and thyme sprigs. Add the steaks, turning to coat and set aside to marinade for at least half an hour (and up to 24 hours for a more intense flavour).
Preheat the oven to 230°C/450°F/Gas mark 8. To prepare the potatoes, boil them in a pan of boiling salted water until tender. Then drain and put on a large baking tin that has been drizzled with a little of the oil. Gently smash each one with a fork, then season with salt and pepper and then sprinkle over the rosemary and thyme. Brush with the remaining oil and then bake for 20-25 minutes until crispy and golden brown.
Lift the steaks out of the marinade and shake off any excess. Cook on a barbecue over medium coals for 2-3 minutes for rare, 3-4 minutes for medium and 5-6 minutes for well done, turning occasionally.
Serve the barbecued T-bone steaks at once on warmed plates with the smashed hot potatoes and spinach salad.