Combine milk, eggs, sparkling water, 1 tbsp melted butter, flour and a pinch of salt to blender.
Blend for about ten seconds. Allow batter to rest for 1 hour. (Batter can be made up to 48 hours in advance).
Heat a sauté (or crêpe) pan over medium heat. Melt approximately 1 tsp butter in pan, or use a spray oil to evenly coat the surface. Pour 1 ladle of batter directly into the center of the pan and swirl to coat bottom.
Cook crêpes for about 30 seconds on the first side, or until the edges start to come away from the pan and crisp up slightly. Flip. Cook for another 10-15 seconds and remove to a covered dish to keep warm.
Spread 1 tsp mustard on bottom 2/3 of crêpe. Add small dab of mustard at edge to seal when rolling. Add ¼ cup spinach to bottom 2/3 of crêpe, over mustard.
Top with two slices of Krakus® ham, cut into thin ribbons. Sprinkle 2 tbsp shredded Gruyère cheese atop ham. Roll crêpes away from you, tucking in ingredients.
Slice in half and serve with freshly chopped chives as garnish (optional).