Step 1

Place the onion slices in a shallow bowl, and cover with vinegar. Add the dill and mustard seeds and let the onions rest in the solution for at least an hour (overnight is fine; store in the fridge). Rinse the onions and pat dry.

Step 2

Heat a large skillet (or two), or griddle, over medium heat. Meanwhile, toss the three cheeses together in a bowl.

Step 3

Butter one side of each of the bread slices. Flick water on the skillet - if the droplets sizzle, the surface is ready.  Note that you'll probably have to prepare the sandwiches in batches.

Step 4

Add the bread slices buttered side up in the pan(s). Let toast for a couple of minutes, then flip (buttered side down). Add a handful of the grated cheese mix to each of four of the toasty bread slices, along with several slices of the pickled onions. Top each sandwich with the other bread slices (buttered side up).

Step 5

Continue cooking the grilled cheese sandwiches, flipping every couple of minutes, until both sides are golden brown. Serve immediately