Start by making the stock: Remove the head and skin of the mackerel and place them in a small pot. Pull off the meat, break it into small chunks, and set it aside. Add the center bone and all the other other bones or trimming to the stock pot. Add the aromatics to the pot, add water, and set on medium heat. Simmer for about 10 minutes, do not boil. Taste the stock and add salt to taste. Pass through a fine sieve and allow to cool slightly.
Whisk the eggs, measure out 375 g of the stock and add it to the eggs. Add about ⅔ of the cheddar and whisk to combine.
Divide the mackerel pieces between 4 small ceramic bowls. Try to use bowls that are not too thick, otherwise, it might take longer to cook. Add the chopped chilis, and ladle over the egg-stock mix.
Set a steaming basket over a pot with boiling water. Place the ramekins in the basket and lower the heat to a minimum to allow gentle steaming. Cover the pot and steam for 17-20 minutes. The Chawanmushi should be just set and still very wobbly. If the Chawanmushi looks and feels like a firm pudding and the liquid separates from the egg, you went too far. You are looking for a very silky consistency with a lot of jiggles.
Top with the rest of the cheddar, the spring onion, radishes, some more pickled chili, and dill sprigs. Finish with black pepper.