Rinse the rice in a fine mesh strainer until the water runs clear, using your fingers to agitate the grains to remove the excess starch (this helps make sure the rice doesn’t turn out mushy and gloppy).
In a small bowl, mix/mash the butter, miso paste, and garlic (if using) until smooth (I start by using a fork to mix, then switch to a small rubber spatula). Reserve about 2 tablespoons of the miso butter to use later. Divide the rest of the mixture on top of each fillet and use your fingers and/or a rubber spatula to spread it around the center of the fillet (this is a bit tricky, just do your best!).
To a medium-large pot with a tight fitting lid (dutch oven works best), add the rice, water, white/light green scallions, and soy sauce and stir to combine.
Cover with a tight-fitting lid, increase the heat to high to bring to a rapid simmer, then reduce to low to maintain a gentle simmer for 5 minutes.
Remove the lid and quickly scatter the veggies evenly over the rice and sprinkle the veg with ½ teaspoon salt. Use a metal spatula to place the fish on top. Cover again and continue to simmer gently for 15-18 more minutes, or until the fish is cooked through/easily flakes with a fork (time will vary if you have a very thin or thick fish, see note).
Meanwhile, to make the salad, add everything to a medium bowl and toss to combine. Set aside.
Carefully remove the fish with a spatula. Turn off the heat and give the rice a gentle fluff to mix the veggies in, then dot the reserved miso butter all over the rice, cover with the lid, and let it rest for 5 minutes.
Scoop rice/veg into your bowl and top with the fish and cucumber salad. Scatter the remaining scallions on top. Squeeze with lemon.