These unique grilled cheese sandwiches have just the right balance of sweet and savory. On the side, an arugula salad is inspired by spring with sweet peas and sliced shallots.
Brush a sheet pan with some oil. Spread sweet potato rounds out in a single layer and brush the tops with some oil. Season with some salt. Roast until tender, 13 to 15 minutes.
Meanwhile, heat a large skillet over medium-high heat. Add cooking oil and then onions to heated oil. Saute onions until very soft, 6 to 8 minutes. Stir in vinegar (portion for the grilled cheese) and sugar and continue cooking until vinegar is cooked off, ~2 minutes more.
While onions cook, in a mixing bowl, whisk together vinegar (portion for the salad), mustard and olive oil. Top with shallots, arugula and peas. (Wait to toss salad until just before serving.)
Wipe out the skillet used to cook the onions and return it to medium heat. Add the first portion of butter and then half the bread slices. Layer bread with sweet potato rounds, balsamic onions and cheese. Top with remaining bread slices and second portion of butter. Cook sandwiches on both sides until golden brown and cheese is melted.