Flank Steak "Caesar Style"

Ingredients

• 2 flank steaks, ca. 650 g and 1,5-2 cm think
• 2-3 tbsp olive oil
• 2 tbsp sea salt
• Freshly ground black pepper
• 1 red onion
• 2 Romaine lettuce hearts
• ½ bunch of curly parsley
• 60 g sliced Parmesan cheese
• 1 fresh egg yolk, medium
• 2-3 tbsp white balsamic vinegar
• 40 ml vegetable stock
• 1 tsp coarse mustard
• 5-6 tbsp olive oil