Combine the mirin and sake in a small saucepan and bring to a boil. Add the soy sauce and honey and reduce the heat to low. Simmer for 2-3 minutes. Set aside and allow to cool.

Place the salmon fillet in a plastic bag and pour the cooled marinade over. Squeeze out any excess air and marinate for between 4 hours and up to overnight.

Preheat an oven to 250°F (120° C).

Use a sheet of parchment paper or aluminum foil to create a little 'boat' (simply crumple up the edges and leave a flat center) to hold the salmon and keep the marinade from running out. Put the 'boat' on a baking sheet, then place salmon fillet in the center. Pour the remaining marinade over the salmon.

Bake for 35-40 minutes, or until the salmon is cooked through but still tender. Remove from the oven and spoon any remaining liquid over the top.

Set the oven to broil and place the salmon back inside. Broil for 2 minutes, or until the surface is glazed and a little browned. Keep an eye on it to avoid burning the salmon. Remove the finished salmon from the oven and set aside to cool.

(Note: you can cook the squash any way you like, but it's best to do it ahead of time, for example while the salmon marinates) Use a potato masher or large spoon to break the cooked squash up into bite sized pieces.

Combine the cooked squash, miso, rice vinegar, sesame oil, and mirin in a bowl. Set aside or refrigerate as-is, or remove a 1/4 cup portion now to make the sauce for serving.

Mix all of the liquid ingredients and toss together with cucumbers in a small bowl or container. Set aside, or refrigerate overnight.

If you're using fresh-cooked rice here (which is ideal), spread it out in a large, shallow bowl or dish and cover with a cloth. Allow the rice to cool until it's a little above room temperature. If you're using rice cooked earlier, cover it with a damp cloth and reheat in the microwave for about 1 minute before continuing as above.

Mix the mirin and rice vinegar in a bowl. Carefully sprinkle this mixture evenly over the rice while using a large spoon or paddle to stir the rice and ensure even coverage. Taste a bit of rice and adjust to taste with more mirin or vinegar as necessary. Cover the rice back up with the cloth and set aside until ready to plate.

To make the sauce, combine the reserved 1/4 cup of the squash mixture with the Japanese mayo and rice vinegar. Mash by hand or with a small mixer/immersion blender. Add water to thin to the desired consistency (1-2 tbsp should do it).

Place a round metal form (like a cookie cutter) on a plate. Fill with 1/2 cup rice, followed by about 1/3 cup of the squash mixture, 3-4 layers of marinated cucumbers, and 1/4 of the salmon. Top with roasted squash seeds, pea sprouts, ikura, and a drizzle of sauce. If you like a bit of spice, add a dusting of shichimi togarashi too.