6 tablespoons unsalted butter
1⅓ cups milk
2 large (or 3 small) eggs
½ cup all-purpose flour
½ cup buckwheat flour
1 leek, white and light green parts only, chopped
1 cup fresh or frozen peas
1 tablespoon chopped fresh mint
1½ cup grated Gruyere cheese (about 6 ounces)
Melt 2 tablespoons of the butter in a small saucepan over low heat (or in a small bowl in a microwave). Put the milk, eggs, all-purpose flour, and buckwheat flour in a blender or large bowl along with the melted butter and ½ teaspoon salt. Blend or whisk until very smooth. Cover the batter and refrigerate for at least 1 hour.
Put 1 tablespoon of the remaining butter in a large skillet over medium heat. When it melts, add the leek and season with salt and pepper. Cook, stirring occasionally, until it softens, 8-10 minutes. Add the peas and continue cooking until they're tender, 3-5 minutes. Remove from the heat and mash the peas and leeks with a potato masher or fork to break them up. Stir in the mint, then taste and adjust the seasoning.
To cook the crepes: Put about 1 teaspoon of the remaining butter in a 10-inch skillet over medium-high heat. When it melts, add enough of the batter to cover the bottom of the pan in a thin but opaque layer. Cook until the top of the crepe is no longer shiny, about 45 seconds, then flip the crepe and cook for about 45 seconds on the other side. Transfer to a paper towel to remove excess butter. Repeat with the remaining butter and batter. (You will use about 3 tablespoons of butter total.)
Reduce the heat to medium-low and return one of the crepes to the skillet. Sprinkle about 3 tablespoons of the Gruyere over half of the crepe, then top the cheese with about 3 tablespoons of the pea mixture. Fold the crepe in half to encase the fillings, and cook until the cheese melts, 2 to 3 minutes. Fold the crepe in half once more and serve. Repeat with the remaining crepes, cheese, and pea mixture.