Spread the mustard on 2 of the 4 sourdough slices.
Place the sliced avocado on top of the 2 mustard bread slices. Sprinkle with salt and pepper.
Place the sliced onions on top of the avocado followed by the cheese slices and the kale leaves.
Cover your sandwich with the other sourdough slice.
Heat your non-stick frying pan and place the sandwiches in there. Cover with a lid and check every 3 minutes if the cheese has melted and the bread is toasted nicely. Flip the sandwich so it's browned evenly from both sides.
Remove from the pan and let cool slightly before eating!