Preheat the oven to 200°C. Put the capsicum on a baking paper-lined tray and roast for 30 minutes or until charred. Remove from the oven, allow to cool for 5 minutes then remove the skin and seeds.
Put the capsicum in a blender with the remaining ingredients and puree until smooth. Season with a little sea salt to taste.
Remove the stones from 4 of the plums (reserve the remaining plums for another use). Puree in a blender or food processor until smooth and set aside.
In a large bowl, combine the fish, lime juice, vinegar, soy sauce and half the sliced red onion. The liquid should completely cover the fish; if not, add a little more. Cover with plastic wrap and put in the fridge for 1½ hours.
To serve, spoon a pool of yellow capsicum sauce on to each plate and surround with a drizzle of plum sauce.
Drain the fish and onion and add to the plates, topping with the remaining onion, the tomatoes, coriander, tobiko and edible flowers, if using. Season with a little salt and add a drizzle of grapeseed oil to serve.