These frittata sandwiches are very flexible. Use any frittata you like. Omit the prosciutto to make it vegetarian or swap with smoked salmon for a nice variation. This is also a great way to use up leftover frittata even when you're flying solo.
Cut each roll in half and butter the halves. Lay a slice of prosciutto on each half. Top the bottom half with a slice of frittata and some greens. Close the sandwiches, wrap in foil or parchment if you're picnicking, and serve at room temperature.