Fill a large pot with water and bring to a boil. Add the rigatoni and cook according to the package's directions.
While the pasta is cooking, melt the butter and heat the olive oil in a large, deep skillet over medium heat. Once the butter is foamy and the oil is shimmering, add the scallions and a healthy pinch of salt. Reduce the heat to low and sweat the scallions for 3-5 minutes or until fairly soft. Remove from the heat and stir in the lemon juice.
Transfer the scallions to a food processor. Add the garlic and blitz until smooth. Pour the puree into the skillet and return to low heat.
Add the yogurt, salt and onion powder to the skillet and stir to combine. Gently heat the mixture until bubbles begin to form on the surface. Turn off the heat.
At this point, the pasta should be done. Drain the rigatoni and add it to the skillet. Toss to coat with sauce.
Divide the pasta amongst four bowls. Add a smattering of arugula, a handful of chopped pistachios, and a healthy sprinkling of shredded cured egg yolk. Serve immediately.