Melt 2 tablespoons of the butter in a large skillet over medium heat.
Add the fennel, onion, ½ teaspoon salt, and ¼ teaspoon pepper, and cook, stirring occasionally, until tender and golden brown, 25 to 30 minutes.
Meanwhile, cook the ravioli according to the package directions. Reserve ¼ cup of the pasta water, drain the ravioli, and return them to the pot.
Gently add in the fennel, reserved pasta water, and remaining tablespoon of butter. Sprinkle with the fennel fronds before serving, if desired.