Spray 9×13 baking dish with cooking spray.
Heat olive oil in medium skillet over medium-high heat.
Add leek and cook until softened, about 5-7 minutes.
Add in chopped spinach and stir, cooking until wilted. Remove from heat.
Layer half of the bread cubes on the bottom of the baking dish.
Top with half of the ham, half of the leek/spinach mixture and half of the cheese.
Repeat with bread, ham, leek/spinach and cheese.
In a medium bowl, whisk together eggs, milk, mustard, salt and pepper.
Pour evenly over baking dish.
Cover with plastic wrap and refrigerate overnight.
In the morning, set out the baking dish for 30 minutes.
Heat the oven to 350 F.
Bake for 45-55 minutes, until golden brown and egg casserole is baked through. Serve and enjoy!