Start by melting the butter on the hob in a small pan, until fully liquified. While the butter is doing its thing, grate your cheeses and mix together.
Once melted, it’s time to assemble the sandwich. Lay out the slices of bread and butter the outside of each slice using a pastry brush, ensuring you butter right to the edges.
This will prevent the sandwich from burning, and also give that beautiful golden brown crunch on the outside.
Next up, flip your bread over, sprinkle the onion on and load up each sandwich with half of the cheese mix. Add another slice of bread on top (butter side up).
One of the key things to remember (and something we all get wrong at some point) is that the butter goes on the outside.
The best way to cook a grilled cheese at home is in a cast iron skillet or a heavy bottom frying pan. You want to cook it low and slow to ensure you get the best crispy crunch on the outside and the cheese melts perfectly.
Place the sandwich in the skillet or pan on a low to medium heat. Keep applying pressure or weight to the top of the sandwich while it’s cooking-this squashes all the filling together, and helps the cheese to melt.
After about 4 minutes, flip the sandwich and cook on the other side. When both sides are golden brown and the cheese is oozing out, you’re done.