Prepare pickled grapes, if using, before making sandwich.

Bring the sliced cheese to room temperature.

In a small bowl, combine mayonnaise, garlic, thyme and pepper; spread mixture on one side of each bread slice.
Place half of the cheese on one bread slice; top with prosciutto, arugula and remaining cheese.
Top with remaining slice of bread, mayonnaise side down.
Butter both sides of the sandwich.

Heat a small skillet over medium heat. Cook sandwich, turning once, for 5 minutes on each side or until golden brown and cheese is melted, adjusting heat as necessary to prevent burning.
Cut the sandwich in half and serve with pickled grapes, sweet grainy mustard, potato chips and fresh-pressed grape juice.

Pickled grapes: Wash and dry ½ lb (225 g) seedless green grapes; place in a 2 cup (500 ml) sterilized canning jar.
Add 4 sprigs tarragon. In a small saucepan, bring 1 cup (250 ml) good quality white wine or champagne vinegar and 1 tbsp (15 ml) granulated sugar to a boil; add a pinch of salt.
Pour liquid over grapes to cover.
Seal the jar and refrigerate.
The grapes will be ready in 3 weeks.