Step 1Toast buckwheat groats in a heavy skillet, over medium heat, until they are lightly browned and toasty, stirring occasionally, about 12 minutes. Keep an eye on them to make sure they don't burn. Remove from heat and set aside to cool. When the groats have completely cooled, grind them on a coarse setting to form grits, so they have the consistency of cornmeal. (One cup of coarsely ground groats will yield about 1 1/2 cups of grits.)
Step 2Place the potato and water in a medium saucepan, cover the pan, and bring the water to a simmer. Cook the potato until it is tender enough to be pierced with a fork, about 25 minutes. Remove the potato but keep the water simmering in the pot. Peel and mash the potato in a bowl with a fork, then return the mashed potato to the pot of simmering water.
Step 3Add the coarsely ground roasted buckwheat grits to the pot. Add one-half teaspoon salt. Bring the mixture to a boil, stirring constantly. Reduce the heat to low and continue cooking, stirring frequently, until the mixture becomes very thick, about 15 minutes. (If it is too thick to stir, add a little more water.)
Step 4While the polenta is cooking, start the sauce: In a large saute pan, saute the leeks in the butter until softened. Reduce the heat and add a pinch of salt. Cover and continue to cook the leeks until very tender, 6 to 8 minutes.
Step 5Stir in the cream, sage and a few grinds of pepper. Gently simmer until the cream reduces and thickens to a sauce-like consistency, about 10 minutes. (If the polenta finishes cooking before the sauce is thickened, remove the polenta from heat, cover and set aside until the sauce is ready, stirring the polenta occasionally.) Taste the sauce and adjust seasoning as desired.
Step 6Stir the grated Parmigiano-Reggiano into the polenta. Serve the polenta in a shallow bowl with the sauce spooned on top. Garnish with Parmigiano-Reggiano.