Bring a large pot of water to a boil. Heavily salt the boiling water and cook ravioli until tender but not overly soft (often 1-2 minutes less than package instructions).
While the water is coming to a boil and the ravioli is cooking, make the brown butter. Warm large saucepan to medium heat and add butter. Once the butter has completely melted, allow it to brown-- butter will foam and sizzle as brown specks appear on the bottom of the pan.
Just as the butter begins to brown, add sage to the pan. Using a spatula, toss the sage in the butter as it cooks, and gently scrape off specks of browned butter from the bottom of the pan. When the aroma of the pan is nutty and the bottom of the pan is a deep toasty-brown color, remove from heat. *
Put the cooked ravioli in a large serving bowl and pour the brown butter and sage on top. Gently toss until pasta is evenly coated with butter. Sprinkle with half of the Parmigiano Reggiano, gently toss again, and then sprinkle the top with the remaining. Serve immediately.
Notes
*There is a fine line between developing a beautiful, deep brown butter and burning your butter. Don't be afraid to let your butter develop deep brown flecks, but be careful not to burn. Cooking at medium heat is the best way to avoid burning.