Gnocchi: In a pot, cover potatoes with 2 inches of cold water. Bring to a boil and cook until easily pierced with the tip of a knife, 20 to 25 minutes. Drain and let sit until cool enough to handle. Peel and quarter potatoes and let cool completely.

Using a ricer or colander, press potatoes into a large bowl. Add flour, egg, and salt to bowl and gently stir until a ragged dough forms, being careful not to overwork. Transfer dough to a floured work surface and gently knead until dough just becomes uniform in color (sprinkling surface and hands with more flour as needed to prevent sticking). 

Divide dough into 6 pieces; roll one out to an approximately 3/4-inch-diameter rope. Using a bench scraper or knife, cut crosswise into 1/2-inch-thick "pillows." Transfer in a single layer to a rimmed baking sheet generously dusted with flour. Repeat with remaining dough.

Sauce: Heat a large skillet over medium. Add pepitas and sesame and poppy seeds; cook, shaking frequently, until pepitas puff slightly and begin to pop, about 2 minutes. Transfer to a plate and season with salt and pepper.

Melt butter in skillet over medium heat, swirling occasionally, until dark golden brown, 8 to 10 minutes. Add sage leaves to brown butter and cook until darkened slightly, about 45 seconds. Remove skillet from heat. Using a fork, transfer sage to plate with seeds. 

Meanwhile, cook gnocchi in a large pot of generously salted boiling water until they float and are al dente, 3 to 4 minutes; drain. Add to skillet with brown butter and return to medium heat. Cook, stirring occasionally, until gnocchi begin to crisp at edges, 3 to 4 minutes. Serve topped with seeds and sage.