To make shortcrust pastry, place the flour and butter into a mixing bowl and rub between your fingers until it has a texture like sand. Then add the egg yolk and enough cold water (about 3 or 4 tablespoons) to make a ball of dough. Add a pinch of salt (as late in the process as you can) and divide the dough into four smaller balls and allow them to rest for 20 minutes.
Line four 4-inch tart shells with a rub of oil or butter and a sprinkling of flour to make them non-stick. Roll each ball of dough out to about a 1 to 2mm circle, and line the inside of each tart shell.
Rest for 20 minutes.
Line the pastry in each tart shell with either a double layer of clingfilm or some greaseproof paper and fill with baking beads. Bake at 180°C for 25 to 35 minutes until golden-brown, removing the baking beads and clingfilm or paper towards the end of the bake. Allow to cool.
Meanwhile, heat half the rapeseed oil in medium-hot pan and fry the onions with a little salt and pepper until just golden, then lower the temperature and cook for a further 45 to 50 minutes. They
shouldn't be too brown, just a light colour and soft texture.
Fill the tarts with the onion mixture and top with slices of the cheese. Place under a low to medium grill until lightly melted.
While the tarts are under the grill, make a dressing by mixing the Arran mustard, honey, vinegar and the remaining rapeseed oil.
To serve, drizzle over the dressing and add a little seasoning, and top with some crisp salad leaves.