Servings: Serves 1
To make mayonnaise: Lightly char rosemary (hold stems with tongs and rotate through burner flame for about 10 seconds), strip leaves and finely chop (yields about 1 Tbsp.). Bring 4 cups water to boil, reduce to simmer and add vinegar. Ease egg into water, cooking until whites set but yolk is still runny, about 2 minutes. Transfer egg (as dry as possible) to blender (or bowl of food processor) with lemon juice and Dijon mustard. Slowly add oils, processing until thick and creamy. Add rosemary. Makes about 1 cup.
To make the sandwich: Gently flake tuna and place in stainless steel bowl. Fold in desired amount of mayo, chopped celery and celery seed, adding salt and pepper to taste. Lay bread on work surface. Spread tuna on two slices and layer with Gruyère. Top with remaining bread slices and press lightly with the palm of your hand to bring tuna just to the edges of each sandwich. Melt a small amount of butter in a skillet over low heat and toast sandwiches until cheese melts and bread is golden brown, about 2 minutes on each side. Transfer to cutting board and cut in half on diagonal. Serve immediately.