Preheat the oven to 400 degrees and line a baking sheet with either aluminum foil or parchment paper. Shmear the butter onto your prepared pan.
Then, make sure your bagel holes are big enough for an egg. Using a biscuit cutter make a hole that is big enough for the egg to fit into.
Crack one egg into each bagel hole. Follow up by sprinkling a bit of kosher salt and cracked pepper over the top of each one to taste and carefully put the pan into the oven.
Bake for about 10 minutes at 400 degrees. If you are adding cheese, make sure to add it once the egg has set, and then just let it get melty before removing from the oven.
Take the pan out of the oven and add any additional toppings you like, then serve immediately.
Notes
Make sure to keep an eye on your cook time. If you want a less cooked egg, start watching your bagels around the 5 minute mark. For the optional toppings: You can cook down the spinach, mushrooms, etc while the bagels are in the oven. Then pile on top when it comes out of the oven.