Preheat oven to 190°C (375°F).
Place the prosciutto pieces on a baking sheet covered with parchment paper and bake for 10 minutes. Remove from oven and set aside.
Preheat barbecue to medium.
Cut up the baguette diagonally. Spread slices with 1 tbsp. olive oil. Place on grill and barbecue 1 minute per side. Set aside.
In a bowl, combine the ranch vinaigrette and blue cheese crumbs and mix well. Set aside.
Preheat barbecue to medium. Oil the grill.
In a bowl, combine remaining oil, lemon zest and lemon juice. Add salt and pepper.
Brush the cut side of the lettuce halves with the oil and lemon mixture.
Grill lettuce, cut side only, for 2 minutes.
Place the grilled lettuce halves on a large serving platter. Drizzle over with vinaigrette and garnish with prosciutto flakes and parmesan petals.
Serve the lettuce with the sourdough bread croutons.