Lemon Thyme Crust Beef Fillet with Dauphinoise Potatoes

Name: Lemon Thyme Crust Beef Fillet with Dauphinoise Potatoes
Ingredients: 25 g (1oz) unsalted butter, plus extra to grease; 1 lg onion, sliced; 1 1/2 floury potatoes; 2 lg garlic cloves, crushed; 500 mL (17 fl 0z) double cream; 350 mL whole milk; nutmeg, freshly grated; half fillet beef, about 950g (2lb 2oz); 1 tbsp. olive oil; 1 tbsp. plain flour; 250 mL (9 fl oz) hot beef stock; 75 mL (3 fl oz) Madeira; 3 slices of white bread, about 125 (4oz), whizzed to breadcrumbs; finely grated zest 1 lemon; small handful fresh thyme, leaves picked; 2 tbsp. Dijon mustard