In a large skillet with a lid, melt 2 tablespoons of the butter over medium heat. Add the onion and half of the garlic. Season with salt and pepper. Cook, stirring often, until the onion is softened and starting to brown, about 3-4 minutes.
Add the kale and season with a bit more salt and pepper. Add 2 tablespoons of water. Cover and cook, stirring occasionally, until the kale is wilted and tender, adding more water as needed if the skillet starts to look dry, about 7-10 minutes. Stir in the lemon juice. Taste and season with additional salt, pepper or lemon, if needed. Transfer to a bowl and cover to keep warm.
Wipe the skillet clean then place it back over medium heat. Add the remaining 2 tablespoons butter. Once melted, add the sriracha and the remaining garlic; cook, stirring, 1 minute. Add the shrimp and season with salt and pepper. Cook, stirring occasionally, until the shrimp are almost cooked through but still pink in the middle, about 2-4 minutes, depending on the size of your shrimp. Scrape the shrimp mixture onto a plate.
Place the skillet back over medium heat (don't wash), and add the coconut milk and sugar. Bring to a boil, and cook, stirring, until slightly thickened around the edges, about 1 minute. Scrape the shrimp back into the skillet and simmer until cooked through. Remove the pan from the heat and season with additional salt, pepper and/or sriracha as desired. Stir in the cilantro and scallions.
To serve, spoon grits into four shallow bowls and divide the kale over top. Spoon the shrimp and sauce over top and garnish with a sprinkle of cilantro and scallions.