Place frying pan over medium-high heat and add olive oil. Allow oil to warm slightly and add your sausage to the pan. (Remove from casings first, if there are any.) Break meat up into crumbles using a wooden spoon and cook through to remove pink, until meat starts to brown. Remove the pan from the heat and set aside, allowing meat to cool slightly.
In a large mixing bowl, add the eggs, half-and-half, dry mustard, salt and pepper and whisk together until combined.
Grate the block of cheddar cheese using the large holes of a box grater until you have two cups.
Tear the bagels into rustic 1-inch chunks. They don't have to be perfect, just about bite sized.
Add half of the cheese (1 cup) and all of the bread into the bowl with the egg mixture. Transfer the sausage into the egg mixture (using a slotted spoon to remove excess grease) and fold together gently until combined.
Coat the 9x13-inch baking dish with cooking spray.
Pour egg, sausage, cheese and bread mixture evenly into the baking dish. Sprinkle the remaining 1 cup of cheese on top.
Cover with foil and refrigerate for 24 hours (at the very least 8 hours). This allows the bread to absorb the egg mixture.
Remove casserole from fridge before cooking and allow it to warm to room temperature before cooking, while oven is heating.
Arrange rack in the middle placement of the oven and heat to 350℉. Bake uncovered until the top is golden brown. To test, insert a knife in the center and ensure that it comes out clean, 50-55 minutes.
Allow to sit for about 10 minutes to set and cool, then slice, serve and enjoy!