Preheat oven to 350 degrees. Put hazelnuts on a baking sheet and toast in oven until light brown and fragrant, about 5-7 minutes. Cool completely and then coarsely chop.
In a small saucepan, melt the butter. When butter starts to sizzle, tear sage leaves into the pan and fry for about 20 seconds. Remove from the heat.
Meanwhile, cook ravioli in 4 quarts of water for each pound. Add a hearty pinch of salt. Bring water to a full boil, reduce heat and then add ravioli. (If you add ravioli when the water is at a robust boil, you risk having the ravioli split open.) Occasionally, stir gently to prevent ravioli from sticking to the bottom of the pot. Once water returns to a gentle boil, cook about 8-12 minutes. Drain carefully, or simply use a slotted spoon to remove the ravioli individually from the pot.
Pour butter sauce over ravioli and sprinkle with toasted hazelnuts. Serve immediately.