Bring a large pot of salted water to a boil over high.
While water comes to a boil, slice scallions, separating the white, pale green, and dark green parts of scallions; set aside.
Heat panko, salt, and 1 ½ tablespoons oil in a small skillet over low. Cook, stirring constantly, until panko is toasted and golden brown, about 4 minutes. Transfer panko mixture to a small bowl; stir in lemon zest. Set aside until ready to use.
Add pasta to boiling water. Cook it according to the package directions for al dente. Stir in spinach by the handful during the last minute of cooking.
Heat the remaining ¼ cup of oil in a large Dutch oven over medium heat. Add white and pale green scallions and garlic; cook, stirring often, until softened, about 3 minutes. Add crushed red pepper and nutmeg; cook, stirring constantly, until fragrant, about 15 seconds. Add feta, chicken stock, and cream to Dutch oven, and bring to a simmer over medium-high. Simmer, stirring occasionally, until slightly thickened and sauce lightly coats the back of a spoon, about 2 minutes.
Add pasta-spinach mixture and peas; cook, stirring constantly, until sauce coats pasta, 1 to 2 minutes.
Remove from heat, and stir in dark green scallion parts, dill, and lemon juice. Add reserved pasta water, 2 tablespoons at a time, if needed to thin sauce to desired consistency.
Divide pasta evenly among 4 bowls, and sprinkle evenly with reserved panko mixture.