Gather all ingredients before starting the recipe.
Pre heat oven to 400°F
Place puff pastry in fridge overnight or let sit for an hour at room temperature before starting. Note that the puff pastry must be cool, if defrosting on the counter, check periodically to make sure that it does not become too soft; if it does, place in fridge for 10-15 minutes before using,
Sprinkle rolling surface with flour and roll pastry to one inch larger than the sheet, or, to a 9 x 12 inch rectangle. Lay the rolled pastry on a cookie sheet lined with parchment paper.
Spread a thin layer of Dijon mustard on the bottom sheet of puff pastry.
Top mustard with 1 lb smoked ham sliced or diced. Leave a 1/2" border on all sides of the pastry.
Add 1/2 lb baby spinach leaves over ham and top with 8 oz shredded white cheddar or gruyere cheese.
Roll out the second sheet of puff pastry to be slightly smaller than the bottom sheet and place this over the ham/cheese mixture. Turn up the edges of the bottom sheet over the top sheet and pinch to make a nice border all around.
Cut 3 slits in the top of the pastry and using a pastry brush, brush the top of the pastry with the egg wash using 1 egg yolk mixed with a tablespoonful of water. If not baking right away, place the tarte in the fridge until ready to bake.
Bake for 30-40 minutes in a non-convection oven, or until the top is nicely browned.
