Use a heavy skillet to toast the buckwheat groats, over medium heat, until they are lightly toasted (stir occasionally), about 12 minutes; remove from heat and set aside so they cool.
Once they are completely cooled, grind them, using a coarse setting, for form grits (they'll have the consistency of corn meal).
Simmer the potato in water until fork tender, about 25 minutes; remove the potato, but keep the water simmering in the pot.
Peel and mash the potato with a fork, then return to the pot of simmering water.
Add the ground buckwheat grits to the pot along with 1/2 teaspoon salt; bring to a boil, stirring constantly, then reduce heat to low and cook, stirring frequently, until mixture becomes very thick, about 15 minutes (if it gets too thick, add a little water; also if it gets done before the sauce, remove from heat and cover).
Meanwhile, saute the leeks, in the butter, in a large skillet, until softened; reduce heat and add a pinch of salt, then cover and cook until very tender, 6 to 8 minutes.
Stir 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.
Stir the grated cheese into the polenta.
Serve the polenta in a shallow bowl, spoon the sauce on top, and garnish with more grated cheese.