They'll indulge you your request for tiny amounts of mackerel, salmon and tuna for you to slice up all pretty (or not so much in my case) at home. I love the combination of the blister-like bursting of the ikura (mmm! nice.) and the velvety soft fish, the citrus-tang of the shiso leaves adding another contrast.
For the rice:
150gr sushi rice
Enough dashi stock to cover the rice, plus up to 1 section of your finger
3 tbsp rice vinegar
1 scant tbsp sugar
1/2 tbsp mirin
1/2 tsp salt
Mix the rice vinegar, sugar, mirin and salt together. Rinse the rice twice, then on the third time gently rub the grains together while rinsing. Add to a heavy bottomed pot with a tight fitting lid and add the dashi stock. Bring to barely a simmer and simmer very gently for 10 minutes. Take off the heat, remove the lid, put a teatowel over the pot and then put the lid back on. Leave for another 10 minutes.
Turn out onto a plate and add the vinegar mixture. Mix the rice using a turning and cutting motion, while fanning the rice to cool it. Perhaps recruit another person to help you out with that bit.
75gr sashimi grade salmon
75gr sashimi grade tuna
40gr salted mackerel
2 tbsp ikura (that's the large salmon roe)
1 tbsp ponzu (soy sauce with yuzu)
4 shiso leaves
Marinated seaweed - this comes in tiny pots and is salty-sweet
Soak the ikura in the ponzu for at least half an hour. Slice the salmon, tuna and mackerel into slices the thickness of a pound coin. I've also seen chirashi with diced fish so you can do that too. Roll the shiso leaves into a cigar shape and slice thinly. Lay the fish on top of the room temperature rice. Add a tablespoon of the ikura and marinated seaweed per serving, and scatter the shiso leaves on top. Serve with wasabi and soy sauce.