Set up a steamer by placing a small rack, (or two sets of chopsticks arranged in a hashtag pattern), into the base of a large 28cm, lidded, saute pan. Place a 20cm cake tin (with a solid base) or heatproof bowl, with about 4cm high sides, on top of the rack (or chopsticks). Wrap the lid in a clean tea towel (this prevents condensation from dropping into the custard) and tie the ends together at the top, then set aside.
Add the tomatoes, maple syrup, vinegar, soy sauce and oil to a small saucepan on a medium high heat. Bring to a simmer, then cover and cook for about 4 minutes until the tomato skins have burst. Remove the lid and simmer on a medium low heat for about 10 minutes more, until reduced and jammy.
Whisk the eggs together in a large bowl. In a separate bowl, whisk together the miso and boiling water and slowly stream it into the eggs, whisking thoroughly until fully combined. Pour the egg mixture through a fine meshed sieve directly into the bowl in the steamer.
Pour enough boiling water into the saute pan until it reaches the bottom of the bowl containing the eggs, and place on a high heat. As soon as it comes to a simmer, cover with the prepared lid and reduce the heat to low and steam for 8-10 minutes until just set, with a slight wobble in the centre (making sure the water stays at a very gently simmer). Check after 8 minutes so as not to overcook the custard. Take the pan off the heat and carefully lift out the bowl.
Take a small knife and score the top of the custard in a 1.5cm criss cross pattern and spoon over the jammy tomatoes. Sprinkle with the chives and furikake and serve.