WATERCRESS, WALNUT AND BLUE CHEESE TART

500g pack puff pastry • 1 tbsp milk • 30g unsalted butter • ½ tsp olive oil • 3 shallots, sliced • 100g watercress, chopped • 2 eggs • 2 tbsp crème fraîche • 80g stilton grating fresh nutmeg • 40g walnuts