To make the crêpes, place dried porcini mushrooms in a spice grinder or mini food processor and pulse until you have a fine powder.

Heat a crêpe pan or small nonstick skillet over medium heat.

Brush lightly with oil.

Stir batter; if it is thicker than heavy cream, whisk in water a tablespoon at a time.

Pour scant 1/4 cup of batter into the pan and swirl to coat bottom evenly.

Cook until batter is set and begins to dry on top, about 2 minutes.

Flip crêpe and cook other side until just browned, about 1 minute more.

Transfer to a plate.

Continue making crêpes, brushing the pan with more oil as needed and stacking crêpes between layers of parchment or waxed paper, until you have at least 12. (If you have more, you can snack on them—yum—or refrigerate up to 2 days for another use.)

To make the filling, melt 1 tablespoon of the butter in a large skillet over medium heat.

Add mixed mushrooms and salt; cook, stirring frequently, until mushrooms are browned, about 10 minutes.

Stir in rosemary.

Transfer mushrooms to a bowl.

Return the skillet to medium heat and melt remaining 1 tablespoon butter.

Stir in flour and cook, stirring, 1 minute.

Stir in half-and-half and stir until thickened.

Return mushrooms to the skillet and stir in nutmeg and 2 tablespoons of the parsley.

Transfer mushrooms to a bowl.

Spoon filling into 12 crêpes and roll closed.

Place in a buttered 9x13-inch pan and bake at 425°F until heated through, about 10 minutes.

Serve garnished with remaining 1 tablespoon parsley.