Heat the oven to 425F. Toss the sweet potato cubes with the olive oil, salt and pepper and spread them in a single layer on a sheet pan with the unpeeled garlic cloves. Roast 25 to 30 minutes, until the edges are properly browned.

Squeeze the roasted garlic out of its skins once it is cool enough to handle.

Melt the butter in a wide skillet over medium heat with the sage leaves and cook 3 to 4 minutes, until the butter smells nutty and the milk solids turn golden brown. Lift out the sage leaves and set them aside to crisp.

Blend the roasted sweet potato, roasted garlic, brown butter, cream, Parmesan, lemon juice and nutmeg just until smooth, about 30 to 40 seconds, then stop. Over-blending makes the sauce gluey.

Cook the penne in well-salted water to al dente, about a minute less than the package says. Reserve 1 ½ cups of the cooking water before draining.

Pour the puree into the skillet over low heat and whisk in the reserved pasta water a quarter cup at a time, usually about 1 cup, until the sauce falls off a spoon in a ribbon and coats the pan when tilted.

Add the drained penne and toss over low heat 1 to 2 minutes so every tube is coated inside and out. Taste and adjust the salt and lemon.

Crumble the crisp sage over the top, add the chopped parsley and more Parmesan, and serve right away.