2 tablespoons unsalted butter, softened
4 thick, pocketless pitas, split
2 tablespoons mayonnaise
1/2 small red onion, thinly sliced
8 ounces thinly sliced pepper Jack cheese
8 ounces thinly sliced rare roast beef
Preheat a skillet or panini press. Grill the sandwiches over low heat until the bread is toasted and the cheese is melted, 10 minutes. If using a skillet, press the sandwiches with a spatula and flip them halfway through. Cut in half and serve right away.