MILAN (Reuters) - European shares fell at the start of a holiday-shortened week on Monday as worries over slowing economic growth and tighter monetary conditions continued to weigh.By 0812 GMT, Britain's FTSE 100 fell 0.6 percent, while France's CAC and Spain's IBEX eased 0.7 and 0.5 percent respectively. Germany's DAX and Italy's FTSE MIB were shut for Christmas Eve."Markets still under pressure from last week's more hawkish Fed update, exacerbating fears about slowing growth and more..