In a small bowl whisk together the Greek yogurt, sriracha and 2 tablespoons of water until completely combined. Cover and refrigerate until ready to serve.
For the green herbed rice:
Bring a large pot of water to a boil over medium-high heat. Add the spinach and cook until wilted but the leaves are bright green, about 1 minute. Remove with tongs to a blender or food processor fitted with the blade attachment with the cilantro, serrano, green onion and ¼ cup water. Puree until smooth and set aside.
Heat a large non-stick skillet over medium heat, add the vegetable oil and heat through. Add the butter and allow to melt, the add the rice and cook, stirring occasionally, until the rice is crispy all over and pale brown in some spots, about 10 minutes. Add 1/3 cup of the green puree and stir to combine.
For serving:
Heat the oven to 400ºF (200ºC). Arrange the sweet potatoes on a medium baking sheet, drizzle with olive oil and season with salt and pepper. Roast until tender, about 25 minutes.
Divide the arugula into serving bowls and top with rice, sweet potatoes, feta and radish (if using). Drizzle with the spicy yogurt sauce and garnish with dill. Serve immediately.