Gnocchi:

Line a large baking sheet with parchment paper. Place sweet potatoes on a plate; microwave on high until tender, about 5-6 minutes per side. Cut in half and cool. Scrape sweet potato flesh into a medium bowl and mash; transfer 1 1/2 cup to a large bowl. Add ricotta cheese; blend well. Add Parmesan cheese, brown sugar, 1 teaspoons salt, and nutmeg; mash to blend. Mix in flour, about 1/2 cup at a time, until a soft dough forms.

Turn dough out onto a floured surface; divide into 3 equal pieces. Rolling between your palms and the floured work surface, form each piece into a 20-inch-long rope (about 1 inch in diameter), sprinkling with flour as needed if sticky. Cut each rope into 20 pieces. Roll each piece over the tines of a fork to indent. Transfer to the baking sheet.

Bring a large pot of water to boil; add 1 tablespoon salt and return to boil. Working in batches, boil gnocchi until tender, 5 to 6 minutes. Transfer gnocchi to a clean rimmed baking sheet. Cool completely. (Can be made 4 hours ahead. Let stand at room temperature.)

Sauce:

In a small sauce pot combine milk, onion, garlic and sage. Place over medium heat and heat until tiny bubbles appear around the edges of the pan, about 5 minutes. Do not boil. Remove pan from heat and let stand for 10 minutes. Remove and discard onion, garlic and sage leaves. Cover to keep warm.

In a saucepan over medium heat, melt the butter. Stir in the flour and cook, stirring, until blended, 1 minute.

Gradually add the milk, whisking constantly. Reduce the heat to low and continue to whisk until the sauce is smooth and slightly thickened, 4 to 5 minutes. Dip a wooden spoon in the sauce to coat it. Pass your finger through the sauce on the back of the spoon—it should leave a clean track.

Strain the sauce into an clean saucepan. Whisk in the shredded Gouda cheese. Cook over low heat, whisking constantly, until the cheese is melted and the sauce is smooth. Season with salt and pepper to taste.

Pour sauce over gnocchi and serve.