In a large skillet, melt the butter over medium heat. Add the onions, thyme and bay leaf; season with salt and pepper. Cook, partially covered, stirring occasionally, until the onions are extremely soft and a light-caramel color, 20 to 25 minutes. Add the wine and cook 1 minute. Add the stock and bring to a boil. Cook 1 minute, then remove from the heat and stir in the creme fraiche.
Meanwhile, heat a cast- iron skillet over medium-high. Coat the steaks with 2 tbsp. EVOO, then sprinkle with the herbes de Provence; season. Using a paper towel, rub the skillet with the remaining 1 tbsp. EVOO. Add the steaks and cook, turning once, about 8 minutes for medium-rare. Let rest 5 minutes.
Slice the steaks against the grain, divide among plates and top with the onion sauce. Serve with the chips, cornichons and watercress. Pass the mustard at the table.