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1
Bring a large, wide pot of salted water to a boil.
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2
Melt 2 tablespoons of the butter in a large skillet over medium heat.
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3
Add the onions, sprinkle with salt and cook, stirring occasionally, until tender, about 4 minutes.
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4
Increase the heat to medium-high, add half the radicchio, sprinkle with salt and pepper and cook, stirring and turning, until tender, about 5 minutes.
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5
Transfer everything to a medium bowl.
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6
Melt 2 tablespoons of the remaining butter in the skillet; add the remaining radicchio, sprinkle with salt and pepper and cook, stirring and turning, until tender, about 5 minutes.
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7
Return the first batch of vegetables to the skillet, stir to combine, remove from the heat and cover to keep warm.
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8
While cooking the radicchio, add the ravioli to the pasta water, stir well and cook until just tender, about 6 minutes.
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9
Reserve 1/3 cup of the pasta water, then strain the ravioli in a colander.
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10
Melt the remaining 1 tablespoon butter in the pasta pot over low heat.
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11
Add the sage and cook, stirring, about 1 minute.
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12
Add the ravioli and toss to coat with sage butter.
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13
Add the vegetables and stir well.
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14
Add 3 tablespoons of the reserved pasta water and toss to coat.
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15
The ravioli should be nicely slicked; if it seems dry, add the remaining water a tablespoon at a time.
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16
Season with salt and pepper.
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17
Divide the ravioli among four warm bowls and serve.
