Sardines have a more assertive fish flavor than canned tuna, which I love, especially in combination with sweet onion and tomato. Grana Padana is very similar to Parmigiano Reggiano, with a saltier and more rustic flavor that stands up to the sardines. You may substitute Parmesan if that’s what you have on hand.
1 cup baby arugula or other baby greens, washed and dried
¼ cup very thinly sliced sweet onion, such as Maui or Vidalia
One 4-ounce can oil-packed sardines, drained well
1½ ounces Grana Padana or Parmesan cheese, shaved with a vegetable peeler
Heat a panini or sandwich press according to the manufacturer’s instructions.
Combine the tomato, lemon juice, olive oil, salt, and pepper to taste in a small bowl
Slice the baguette pieces in half lengthwise and spoon the tomatoes on the bottom half of each baguette. Arrange some of the greens on top of the tomatoes. Arrange the sliced onion on top of the greens. Arrange the sardines on top of the onions. Top with the cheese. Top each sandwich with the top half of the baguette.
Put the sandwiches on the press, pull the top down, and cook until they are browned and crisp, 4 to 7 minutes, depending on how hot your machine is. Carefully remove from the press and serve immediately.