Whether it's a homemade or store-bought blend, the seasoning provides your grilled cheese with a combination of garlic, poppy seeds, sesame seeds, onion, and salt.
Depending on the bread and cheese you use, you can either add everything bagel seasoning into the melting cheese or crust your bread with it using a binder such as butter.
If you prefer to make your grilled cheese with an actual bagel, you can do so by inverting the sliced halves so that the seasoning on top doesn't spill out.