Grease a 9x13 inch casserole dish and set aside.
Add the olive oil to a pan over medium-high heat. Next, add the sausage and leeks. Break up the sausage into small pieces and cook until the sausage is fully browned and the leeks have softened, about 10 minutes. Set aside for now.
Add the cubed bread into the casserole and top with the sausage and leek mixture. Toss everything to combine and set aside.
Into a large bowl, add the eggs, salt, pepper, chives, cheese and milk and whisk everything until fully incorporated.
Pour the egg mixture evenly into the casserole dish, making sure to coat all the bread. If needed, press the bread down into the egg mixture using the back of a wooden spoon.
Cover the casserole with aluminum foil and refrigerate overnight.
*Note: You can bake the casserole after assembling, just wait at least 20 minutes for the bread to absorb the egg mixture, and bake as normal.
To bake the casserole, preheat the oven to 350 degrees F and place the casserole, covered in aluminium foil, into the center of the oven. Bake for 50 minutes.
After 50 minutes, remove the foil, increase the heat to 375 degrees F and bake for another 10-15 minutes, or until the top is golden brown. You will know it is fully cooked when you don't see any liquid when you press into the center of the casserole.
Let the casserole cool for 10 minutes, then garnish with extra chives. Serve warm and enjoy!