Apple-Gruyère French Toast With Red Onion

1 3/4-inch-thick slices challah bread from middle of loaf

grated Gruyère cheese (about 3/4 cup)

small red onion, very thinly sliced

large eggs, lightly beaten

whole milk

kosher salt

ground black pepper

extra virgin olive oil

small apple, cored, quartered and very thinly sliced

Fried eggs, for serving, optional