Broccoli Rabe Panini with Mozzarella

1 pound (455 grams) broccoli rabe
1 tablespoon butter
1 tablespoon olive oil, plus more for brushing bread
½ medium yellow onion, diced small
Salt and freshly ground black pepper
1 medium garlic clove, minced
Pinch of red pepper flakes
Four ¼-to-½-inch slices of a country bread; or two 4-inch lengths from a flattish ciabatta loaf, halved widthwise
¼ pound (115 grams) mozzarella, thinly sliced

Remove and discard the broccoli rabe’s heavy stems. Cut the leaves into ½-inch ribbons. Leave any small florets whole, but halve any larger ones.
Heat the butter and 1 tablespoon olive oil in a large sauté pan over medium heat. Add the onion and a pinch of salt and pepper, and cook until onion is a bit brown at the edges, about 5 to 7 minutes. Increase the heat to medium-high and add the prepared rabe, the garlic, and pepper flakes, and sauté until the greens are tender, about 4 minutes. Season with salt to taste. You should have about 1½ cups of this mixture.
Heat a well-seasoned ridged grill pan over moderate heat, or a panini press according to manufacturer’s instructions. Brush one side of each slice of bread, or the outsides of your ciabatta sections, with olive oil. Arrange one piece of bread, oiled side down, on grill pan or panini press. Spread with half of rabe mixture and half of mozzarella slices. Arrange second slice of bread, oiled side up, on top. Lower panini-press lid—or an additional frying pan—onto the top of the sandwich, and cook for 4 to 8 minutes, until nicely toasted underneath. Carefully flip sandwiches, and grill on the other side for 3 to 5 minutes.
Cut the sandwiches in half, and eat while still hot.