Beef and Cauliflower Pie

Name: Beef and Cauliflower Pie
Ingredients: 5 cups cauliflower; 1 onions; 11 button (white) mushrooms; 1 tbsp olive oil; 340 g ground beef, lean; 1/2 cup canned tomatoes (diced); 4 tsp tomato paste; 2 tsp Worcestershire sauce; 2/3 cup beef broth; 2/3 cup frozen peas; 1/2 cup milk, partly skimmed, 2%; 4 tsp butter, unsalted; ground pepper to taste; 1 pinch salt