<i>Makes 6 to 8 servings</i>
2 pounds sweet potatoes, preferably yellow-fleshed
1 pound Russet potatoes
—White pepper in a mill
2 cups all-purpose flour, plus more as needed
3 tablespoons butter, at room temperature
3 ounces (3/4 cup) grated dry Jack or Asiago cheese
1 pound young spinach leaves
1 garlic clove, pressed
1 teaspoon, plus more to taste, sweet Spanish paprika
3 tablespoons toasted and chopped pecans
Preheat the oven to 350 degrees.
Use a fork or a paring knife to puncture the sweet potatoes and Russet potatoes in several places. Set them on the center rack of the oven and bake until fully tender when pierced with a fork or bamboo skewer; do not undercook them.
Remove from the oven and let cool for about 10 minutes.
Using oven mitts, break the potatoes and sweet potatoes in half crosswise. Press each half through a potato ricer into a medium mixing bowl, removing the skin after each press. If you do not have a potato ricer, remove the skins and pass through a food grinder or mash with a fork until quite smooth.
Season very generously with salt; use at least 2 teaspoons. Add several turns of white pepper.
Add the flour 1/2 cup at a time and mix well after each addition.
The dough should be very smooth, soft and just a bit sticky.
Dust a clean work surface with flour, divide the dough into 4 or 5 pieces and use your hands, dusted with flour, to roll the dough into long ropes about 3/4 inch in diameter. Cut each rope into 3/4-inch pieces.
Let the dough rest briefly.
Fill a large saucepan two-thirds full with water, season with salt and bring to a boil over high heat.
Take a fork with long, rounded, slim prongs. Hold the fork sideways, parallel to your countertop or work surface, with the concave side facing toward you. With your other hand, place a gnocchi on the inside curve of the fork just below the points of the prongs and press it against the prongs with the tip of your index finger perpendicular to the fork. While pressing the dumpling with your finger, flip it away from the prong tips and toward the handle of the fork. Don't drag it, flip it. As it rolls toward the base of the prongs, let it drop to the counter. The dumpling will be somewhat crescent shaped, with ridges on one side formed by the prongs, and a deep depression on the other formed by your fingertip.
Set a warmed serving dish next to the stove, put the butter and cheese into it and cover with a clean tea towel. Rinse the spinach in cool water and, without shaking off the water, set it in a large saute pan or wok.
To cook, drop 10 to 20 gnocchi into the boiling water; you don't want to crowd them so adjust the quantity based on the size of the saucepan. When the gnocchi rise to the top of the water, cook for another 15 to 20 seconds, use a slotted spoon to lift them out, shake off excess water and set in the serving dish. Cover with a tea towel and continue until all gnocchi are cooked.