Whisk the buckwheat flour, all-purpose flour, and salt together in a mixing bowl. In a second bowl, whisk the eggs, milk, and water until smooth.
Pour the wet ingredients into the dry and whisk until the batter is smooth and pourable, close to the consistency of heavy cream. Cover and rest 20 minutes at room temperature.
Heat a 26 cm / 10-inch nonstick skillet over medium heat and brush with a thin layer of butter.
Ladle about 60 ml batter into the pan and swirl to coat in a thin layer. Cook 1 to 2 minutes until the edges lift and the surface looks dry, then flip and cook 30 to 45 seconds more.
Stack the cooked crepes on a plate, greasing the pan lightly between each. Thin the batter with a tablespoon of water halfway through if it thickens. You'll get about 8 crepes.
While the crepes cook, melt the butter in the 16 cm saucepan over medium heat with the sage leaves. Cook 3 to 4 minutes until the butter turns golden and smells nutty, then take it off the heat.
Lay each warm crepe flat and scatter grated cheese over one half. Fold in half, then in half again.
Return the folded crepes to the warm skillet, cover, and heat 1 to 2 minutes until the cheese is fully melted.
Transfer to plates, spoon the sage butter over the top, and finish with a grind of black pepper. Serve right away while the cheese is still molten.