Cook your rice or rice noodles (rice cooking instructions). Cut the cod into 4 filets. Pat dry with a paper towel, then sprinkle all over with salt to lightly coat and season the fish. Set aside.
In a large skillet (that has a lid), warm the oil over medium heat. Add the white/light green parts of the scallions and ¼ teaspoon salt. Cook for about 2 minutes or until softened. Add mushrooms, carrots, ginger, red pepper flakes, and ½ teaspoon salt. Cook for 2 minutes or until the veggies have softened a bit, stirring occasionally.
Stir in the coconut milk, lemon slices, and remaining ¼ teaspoon salt. Nestle the fish filets into the skillet so that they are about half covered with the liquid.
Cover the skillet with a lid, bring to a boil over high heat, then reduce the heat to low and simmer for about 5-8 minutes or until the fish flakes easily with a fork (times vary greatly depending on fish thickness. If some filets are done sooner, remove them and continue to cook the rest).
Once the fish is cooked, transfer it to a plate. If adding greens, submerge them into the coconut milk now, cover, and simmer until they are wilted, less than a minute. Turn off the heat.
In a jar or liquid measuring cup, add the miso paste and scoop some of the coconut milk into it. Use a fork to mix/mash it up until mostly smooth, then stir back into the skillet.
Add rice or noodles to your bowl. Top with fish and veggies and spoon more coconut milk sauce over everything. Top with the green parts of scallions, any other herbs (if using), chili oil (if using), and a squeeze of lemon.