Cut each of your slices of bread into four triangles and line the bottom of an ovenproof dish with half of them.
Cover the bread with the mushrooms, then onion. Sprinkle with quarter of the cheese, add the ham or bacon, then sprinkle with a further quarter of the cheese.
Finish with another layer of bread, and sprinkle with the remaining cheese.
Beat together your eggs and mustard, then slowly add the milk and whisk together. Pour over the bread.
Bake for 45 minutes, or until your cheese bake looks puffy and golden. Sprinkle with the sliced spring onion and chili. Mix together some mayonnaise and sriracha to taste, and serve with salad or baked beans.