To make saffron oil, gently heat oil in a medium saucepan until it is just warm, about 110 degrees.
Place the saffron in a very small cup and squeeze the lemon juice on top. Let sit for about 5 minutes to allow the saffron to bloom.
Remove the oil from the heat and whisk the saffron lemon juice mixture into oil. Let cool. It’s best to make this the day before so the oil develops a rich color and flavor.
To make avocado purée, cut the avocado in half, remove the pit, and set aside. Scoop the flesh out of the skin and place in a medium mixing bowl. Squeeze in the lemon juice. Using a microplane, grate the garlic into the bowl, and season with salt. Whisk the ingredients together until you have a smooth purée, then transfer it to a pastry bag, along with the avocado pit, to ensure it remains bright green.
To serve Squeeze about 2 oz. of avocado purée into the center of an individual bowl. Drape the kampachi slices over the purée, and drizzle the saffron oil around the fish and the avocado. Garnish with radish and nasturtium leaves. Finish with flaky sea salt.