In a large Dutch oven or other large heavy pot with a tight-fitting lid, combine the rice, coconut milk and 2 cups of water; season with 1 teaspoon salt. Bring to a boil, covered, over high.
Meanwhile, in a medium bowl, combine the scallions with the miso, soy sauce, olive oil, turmeric and a few grinds of pepper to form a chunky paste. Add the salmon; toss to coat.
When the rice starts to boil, reduce the heat to medium-low, adjusting it as needed to maintain a simmer. Stir to make sure nothing is sticking on the bottom.
Layer the spinach on top of the rice. Squeeze 2 lime quarters over the spinach. Nestle the salmon pieces on top in an even layer, scraping in any scallions remaining in the bowl. Cover and cook until the salmon is just cooked through, breaking apart the thickest piece to check, 12 to 16 minutes.
Squeeze the remaining lime quarters over the salmon. Top with cilantro, if using. Scoop into bowls or plates to serve.
