This is a French appetizer called Petits Pissaladieres, it is easy to make and has extreme flavor, if you or your guests are not an anchovy lovers, then I advise you not to make this. The onion mixture can made a day in advance, cover and chill. Reheat over medium heat, stirring about 10 minutes until heated through...or also the puff pastry squares can be baked a day in advance, cooled completely and kept in an airtight container at room temperature, just reheat in a 350°F oven for 6 minutes before topping. These squares are absolutely wonderful, I have served them many times in the past, trust me, they are worth the time to make! This recipe will make about 35-36 mini tartlets, if you are serving a large group of people then I strongly suggest to double the recipe, as people will be eating three, four or more at a time, they are THAT good! Cooking time is for both pastry and onion topping.