Preheat oven to 325° and ensure rack is set to bottom setting.
Slice bacon into thin strips and fry until browning and just crispy. Remove as much fat as possible from pan, leaving bacon in pan.
Rinse leek well and slice lengthwise in half. Place flat side down and slice into half circles.
Roughly chop spinach. Add to pan along with leek and saute until slightly softened. Remove mixture to paper lined plate and drain well, squeezing out any extra moisture from spinach and any remaining fat from bacon.
Arrange in bottom of pie crust. Shred cheeses and spread evenly over top.
Whisk together eggs, milk, Dijon, salt and pepper. Place pie shell onto a baking sheet and carefully pour custard into pie shell.
Bake on lower rack of the oven for 35 minutes or until just set in the middle. Allow to rest for 10 minutes before slicing.