Place the flour in a mixing bowl, add the salt and egg. Mix well. Gradually add the water while stirring to avoid lumps. Finish by adding the buttermilk. Beat the batter, cover, and refrigerate for at least 2 hours.
Heat a nonstick skillet over high heat and butter it. Pour in a ladleful of batter. After about 1 1/2 minutes, when the edges start to lift, flip the crêpe with a spatula and cook for about 1 more minute on the other side.

For the savory filling: Thinly slice the onions and mushrooms, then sauté them in olive oil in a large skillet until tender and lightly golden. Add the liquid from the mackerel cans (set aside the fillets), let it reduce, season with salt and pepper, then add the cream and mix. Keep warm.

For the sweet filling:
Wash, core, and cut the apples into quarters, then slice them thinly. Sauté the apples in a skillet over medium heat, covered, with a pat of butter for about 5 minutes. Once the apples are softened and slightly colored, add 1 1/2 tablespoons honey and the cinnamon.
Let the apples caramelize, stirring occasionally, until they are golden brown. Set aside.

Reheat the crêpes. Add the fillings and serve with a salad.