These crepes are best if the batter is made the day before.
Make the crepe batter: Whisk all ingredients together until smooth. For best results, refrigerate overnight. Mixture will keep in refrigerator for up to 3 days.
Make the béchamel: In a saucepot over medium heat, melt butter. Stir in flour and onion and cook until lightly browned. Lower heat and stir in cream and milk. Cook over low heat until slightly reduced. Stir in cheeses until melted, then add dijon mustard and blend mixture with an immersion blender. Season with salt, pepper, and nutmeg. Set aside in refrigerator to cool.
Make the crepes: Preheat broiler to 400 degrees or low setting. Transfer béchamel to a piping bag (or a zip-top bag with a corner snipped). Heat a pan until lightly smoking. Spread a thin layer of batter over the surface. Pipe a ring of béchamel into the center of the batter, leaving enough room in the center for an egg. Crack an egg into the ring and place 3 rectangles of Gruyère around the egg, forming a triangle and leaving the egg exposed.
When edges of crepe have browned, fold each side inward over the Gruyère, still leaving the egg exposed. Then, fold each corner of the triangle inward about an inch.
Transfer folded crepe to the oven and bake until egg has set. Finish with a pinch of salt, pepper, and fines herbes over the egg. Repeat with remaining batter. Serve with bacon.