Cauliflower Topped Shepherds Pie

Name: Cauliflower-Topped Shepherd's Pie
Ingredients: 1/2 head cauliflower, florets only; 2 medium butter potatoes; 1 teaspoon olive oil; 1/2 large onion, chopped; 1 pound ground lamb; 1 cup peas; 2 tablespoons flour; 3 tablespoons Worcestershire sauce; salt & pepper; 1/2 cup half & half; 4 tablespoons butter; 1/3 cup parmesan (plus extra for the top); 1 pre-baked pie crust