5 Very Thin Slices Prosciutto, each cut crosswise into thirds
15 Paper-thin Lemon Slices
Butter, room temperature
2 Tbl.(or more) Butter
Mix tuna, mayonnaise, onion, pickle and mustard in medium bowl. Season with S & P.
Divide half of Fontina cheese among 15 bread slices, trimming cheese to fit. Cover each with 1 piece of prosciutto,1 lemon slice 1 Tbl tuna mixture and remaining cheese slices. trimming cheese to fit. Top each with 1 remaining bread slices. Generously spread both sides of sandwiches with butter.
Melt 2 Tbl. butter in skillet over medium-high heat. Add sandwiches in batches and cook until cheese melts and bread is golden brown, adding more butter as necessary, about one minute per side.