SHALLOT OLIVE AND CHORIZO TART

10g unsalted butter • 1 tbsp olive oil • 400g echalion shallots, peeled and quartered • 2 tbsp sherry vinegar • 250ml fresh Cooks’ Ingredients Vegetable Stock • ½ tbsp maple syrup or brown sugar • 3 thyme sprigs • ½ x 350g tin Fragata Stoneless Manzanilla Olives Filled with Chorizo, drained • 320g sheet shortcrust pastry • 1 medium British Blacktail Free Range Egg, beaten for glazing • ¼ tsp nigella seeds (optional)