Fry the onions in the butter, add the meat until it is lightly golden. Add the tomatoes, chilli, bay leaf, merquén and salt; cook over a low flame for between 30 - 35 minutes, stirring occasionally, until the majority of the liquid has evaporated. Remove from the heat, discard the bay leaf and add the olives. Put three spoonfuls of filling (when cool enough the handle) in the middle of each empanada skin and fold over in the middle; press the edges together with your fingers to form a half moon shape closed sealed around the edges and filled inside. Use a fork on the edges to fully seal them. With a fork carefully pierce a couple holes in the top side of the empanada. Using a pastry brush, brush some egg wash (the mixture of egg yolk and water) on the top side of each empanada. Cook in the oven at 400° for 20 to 25 minutes or until they are golden.