In a saucepan over medium-high heat, bring the water to a boil. Add the grits slowly, stirring to prevent clumping. Reduce the heat to low. Cook, uncovered, stirring from time to time, until most of the water has been absorbed, 20 to 25 minutes. Season with salt and pepper.
While the grits are cooking, in a frying pan or sauté pan over medium heat, melt 4 table- spoons of the butter. When it foams, add the greens, stir quickly and then add a few tablespoons of water. Cover with a tight-fitting lid, reduce the heat to low and simmer until the greens are soft, tender to the bite, and most of the water has evaporated, about 15 minutes.
In a frying pan or sauté pan over medium heat, melt the remaining 4 tablespoons butter. When it foams, add the onion and cook, stir- ring, until translucent, 2 to 3 minutes. Add the mushrooms, stir well, reduce the heat to low and continue to cook until the mushrooms are tender and most of the juices they released have been absorbed, about 5 minutes. Season with salt and pepper.
To serve, divide the grits among warmed individual soup plates. Spoon the mushrooms onto the center of the grits and then top with the greens. Serve immediately.
Adapted from the Davis Farmers Market Cookbook by Georgeanne Brennan and Ann M. Evans (Mirabelle Press, 2012)