Prepare the Crepe Batter:

1. In a large mixing bowl, whisk together the flour and salt.
2. Gradually add the milk and sparkling water, whisking constantly to avoid lumps.
3. Beat in the eggs one at a time until the batter is smooth.
4. Stir in the oil and let the batter rest for 15 minutes to ensure tender crepes.

Cook the Crepes:

1. Heat a non-stick skillet over medium heat and lightly grease with oil.
2. Pour a ladleful of batter into the skillet, swirling it to coat the bottom evenly.
3. Cook each crepe for about 2 minutes on each side until golden brown.
4. Repeat with the remaining batter, stacking cooked crepes on a plate.

Prepare the Filling:

1. Heat oil in a large skillet over medium heat.
2. Add the diced onion and sauté until translucent.
3. Stir in the sliced mushrooms and minced garlic, cooking until the mushrooms are tender and their juices are released.
4. Reduce the heat and add heavy cream, cheddar cheese, and fresh spinach.
5. Stir until the spinach wilts and the cheese melts. Season with salt and black pepper to taste.

Make the Creamy Cheddar Sauce:

1. Melt butter in a saucepan over medium heat.
2. Stir in the flour to create a roux and cook for 1 minute.
3. Gradually whisk in the milk, stirring to keep the sauce smooth.
4. Add garlic powder, nutmeg, salt, and black pepper. Continue stirring until the sauce thickens.
5. Fold in the shredded cheddar cheese until fully melted.

Assemble the Crepes:

1. Lay each crepe flat and spoon a generous amount of the mushroom-spinach filling into the center.
2. Fold the crepes over the filling and arrange them in a baking dish.

Bake and Serve:

1. Preheat the oven to 180°C (350°F).
2. Pour the creamy cheddar sauce over the crepes, ensuring they’re well coated.
3. Sprinkle Parmesan cheese on top.
4. Bake for 10-15 minutes, or until the sauce is bubbly and the top is golden brown.
5. Garnish with fresh chives and serve warm.