Ruth Reichl’s Grilled Cheese

So, i saw a video on facebook with a grilled cheese sandwich recipe and I had to try it.
It's delish and soo easy to make

butter, 2 slices thickly sliced, sturdy sourdough bread, mayonnaise, any combination of shallot, leek, scallions, onion, 1 clove garlic, 1/4 pound cheddar cheese, divided

Chop finely the onion mix and the garlic, add it to the grated cheese.
Butter the bread, mount the cheese on top. Smear the outside of the bread with mayonnaise and add a little cheese.
Fry the bread on both sides for a couple of minutes each.
Serve while it's hot.