Spread mustard on baguette bottom; layer with pear, prosciutto, and Gruyère. Top with baguette top; cut into 4 equal sections. Brush sandwiches all over with mayonnaise. Heat grill pan over medium. Add sandwiches; weigh down with heavy skillet. Cook until cheese melts and bread is toasted, about 1 1/2 minutes per side.
One, Two, Three... Aioli!
For a quick, fancyish flourish for sandwiches, fish fillets, or fries, stir some fresh tarragon and whole-grain mustard into mayo to taste, then spread, dollop, or dip at will!