Heat the oil in a large frying pan over high heat.
Add the onion and cook, stirring until browned, then add the kimchi.
Reduce the heat to medium.
Once the kimchi juice has reduced, stir through the gochugaru, sesame oil, honey, soy sauce and a pinch of black pepper until well combined, then remove from the heat.
Heat a second pan or char grill pan over low heat.
Butter the sliced bread on one side, then place half the slices (buttered side-down) into the pan.
Divide the kimchi mixture, cheeses, cabbage and spinach between the bread slices, then top each with a second piece of sourdough, butter-side up.
The toasties are ready once the bread is browned (turn carefully once during cooking), and the cheese is melted.
