Combine ground beef, onion, garlic, salt, and pepper in a skillet over medium heat. Cook and stir until beef is browned and crumbly and onion is translucent, about 10 minutes. Drain off excess fat.

Transfer beef mixture to a large bowl; mix in ricotta cheese, Parmesan cheese, parsley, and egg until thoroughly combined.

Line a baking sheet with parchment paper and lightly dust with flour. Place 15 wonton wrappers onto a lightly floured surface.

Spoon about 1 tablespoon filling into the center of a wrapper. Dip your index finger into beaten egg yolk and smear a light coating of yolk all around the edge of the wrapper. Place an unfilled wrapper on top of the filling and lightly press the two wrappers together. Pick up the ravioli gently in your hand and pinch the edges together tightly, squeezing out any air. Seal and crimp the edges with a fork, then place onto the prepared parchment paper. Repeat to make remaining ravioli.

Bring a large pot of lightly salted water to a boil. Cook ravioli in the boiling water until tender yet firm to the bite, 1 to 3 minutes. Drain and serve.