How to Make It
Put the steaks in a large, shallow glass or ceramic baking dish in a single layer. Pour in 2 cups of the Banyuls, add the thyme sprigs and turn the steaks to coat. Cover the baking dish and refrigerate for 3 hours.
Bring a medium saucepan of water to a boil. Add the pearl onions and cook for 2 minutes to loosen the skins. Drain the onions and peel them.
Melt 2 tablespoons of the butter in a large skillet. Add the pearl onions, water and sugar, cover and cook over low heat until the onions are tender and most of the water has evaporated, about 30 minutes. Uncover the skillet and season the onions with salt and pepper. Add the remaining 2 tablespoons of Banyuls and cook over moderate heat, shaking the pan, until the onions are caramelized and coated with a syrupy sauce, about 3 minutes. Remove from the heat.
Light a grill or preheat a grill pan. Pour the marinade, along with the thyme sprigs, into a small saucepan and boil over moderate heat until reduced to 1/2 cup, about 10 minutes. Discard the thyme sprigs. Remove the saucepan from the heat and whisk in the remaining 4 tablespoons of butter, 1 tablespoon at a time, until the sauce is thick and smooth. Whisk in the olive oil and season with salt and pepper.
Season the steaks with salt and pepper. Grill the steaks over a moderately hot fire or moderately high heat until deeply charred and rare in the center, about 4 minutes per side. Put the steaks on a large warmed platter.
Reheat the pearl onions. Rewarm the wine sauce over low heat, whisking constantly. Pour the sauce into a warmed gravy boat. Arrange the steaks and pearl onions on dinner plates and serve. Pass the sauce at the table.