Preheat the oven to 350ºF and grease the inside of a 9x13 baking dish.
Tear the croissants into large pieces and place into the bottom of the baking dish.
Repeat this process for the ham, tearing into large pieces and placing in between the croissant pieces, packing the baking dish fully.
Top croissants and ham pieces with the slices of butter.
In a medium mixing bowl, mix together milk, heavy whipping cream, mustard, eggs and cheese (reserving a handful of cheese for garnishing).
Pour mixture over the croissants, distributing evenly.
Top with the remaining cheese and poppy seeds.
Bake for 45-55 minutes or until the liquid has cooked thoroughly and thickened.