Grilled striploin sandwich with Boursin and caramelized onions

Ingredients

• 1 large yellow onion, halved and thinly sliced
• Olive oil
• 1/2 tsp (2.5 ml) dried thyme
• 2 tbsp (30 ml) white balsamic vinegar
• 1 striploin steak (300 g / about 1.5 inches thick)
• 2 tbsp (30 ml) Herbes de Provence
• 2 tsp (10 ml) coarse salt
• 1 tsp (5 ml) black pepper
• Half a ciabatta-style baguette, sliced lengthwise
• Boursin Cuisine Garlic & Fine Herbs (to taste)
• Fresh arugula (to taste)