1. Preheat your oven to 200°C/Fan 180°C/Gas 6. Fill and boil your kettle. Tumble the shallots into a large heatproof bowl and cover them with hot water from the kettle. Leave to stand for 5 mins – this will make them easier to peel.
2. Drain the shallots and let them cool for a few mins, then trim the ends and peel off the soft brown skins. Leave any smaller ones whole and cut large one in half or quarters. Tip them into a ovenproof frying pan, cake tin or roasting tin (see our tip below on picking the perfect pan for this tart).
3. Drizzle the shallots with 1 tbsp olive oil and sprinkle over a little salt and pepper. Slide into the oven to roast for 15 mins.
4. After 15 mins, remove the shallots from the oven and dot over the butter. Drizzle over the maple syrup and balsamic vinegar. Scatter over most of the thyme leaves, saving a pinch for garnishing later. Slide the pan back into the oven for 20 more mins.
5. While the shallots roast, remove the pastry from the fridge. Place the pastry between two sheets of baking paper and roll into a thin square, rectangle or circle, slightly larger than your dish. Alternatively, dust your work surface with a little flour, if you have some, and roll out the pastry on that. Dollop on the wholegrain mustard and spread that over the pastry.
6. Remove the caramelised shallots from the oven. They should look glossy, slightly pink and soft. Carefully drape the pastry, mustard-side down over the shallots and cut away any excess pastry. Tuck the edges into the dish, folding and pinching the pastry around the shallots to create a crust (be careful not to burn your fingers on the pan). Pop the pan back into the oven for a final 30 mins baking.
7. After 30 mins, remove the tart from the oven. The pastry should be golden and slightly puffed up. Leave to rest for 10 mins then carefully run a knife around the edge of the pastry to loosen it from the tin. Place a board or plate that is larger than the tart on top. Flip the plate and tin over. Carefully lift the pan away from the board or plate. The tart should slip out onto the plate.
8. Using a vegetable peeler or really sharp knife, remove the rind from the Saint Giles cheese, then peel or slice wafer thin slices off the cheese. Scatter the cheese over the tart - it will melt slightly on the warm tart. Scatter over the remaining thyme leaves. Serve in slices.
9. What Pan?
You can use any small roasting tin, baking tray, cake tin or ovenproof frying pan. The shallots should fit snugly in the tin or pan in a single layer – a 20cm-wide frying pan is a good size for this tart.
10. Keep on rolling
No rolling pin? You can use a wine bottle instead.