Mornay sauce Whip the butter, then add sifted flour while mixing well with a whisk. Stir constantly while cooking at a medium heat for a few minutes. Add half of the milk (pre-heated) while whisking vigorously. When the mixture thickens, add the rest of the milk and cook for a few more minutes. Add the grated cheese and make sure it is completely mixed in. Remove from heat, cover with plastic wrap touching the top and let cool to below 65°C (150°F). Then add the egg yolk and mix well. Set aside in the refrigerator. When completely chilled, put half of the mixture in a pastry bag with a wide ruffle tip. Then add the finely chopped chives to the other half.
Plating Spread the Mornay sauce that contains chives on the slices of ham, roll them up and wrap tightly in plastic wrap. Put in the refrigerator. Toast the slices of bread in a toaster or grill in a non-stick pan with a little olive oil.
Cut the rolls of ham into pieces 2 cm thick and place between two slices of bread. Pipe the remaining Mornay sauce in the pastry bag on top and brown in the oven broiler.
Garnish with the herbs and flowers.