Melt the stick of butter in a pan, over low heat. Brush surface of a large, flat baking dish with butter. Take 1/2 of your phyllo and lay it on the baking dish - brushing each sheet with butter as you lay it out. Use the remaining 1/2 phyllo to fashion "sides" for the tart, by rolling them into cylinders and placing them on the edge of the tart base (see photo). Remember to brush each sheet of phyllo as you go.
Into the remaining butter, add the sliced onions and cook over very low heat until caramelised. This can take anywherer from 10-45 mins.
Place the sliced pears or figs on the pastry, top with the caramelised onions, the crumbled blue cheese and thyme. Splash with balsamic vinegar or reduction and bake at 180 for 12 mins or until the sides are golden.