Combine the cabbage, 1/4 cup water, the olive oil, salt, pepper, and mustard seeds in a small saucepan and heat over medium-high heat until the water comes to a boil. Turn the heat to medium-low, cover, and steam, stirring occasionally, until the cabbage is very soft and completely wilted, 10 to 15 minutes. Use a slotted spoon to transfer the cabbage to a bowl, leaving any excess water in the pot, and set aside.
Heat a panini or sandwich press according to the manufacturer's instructions.
Spread some butter on one side of each slice of bread. Put the bread, buttered sides down, on a cutting board. Spread mustard on two of the slices. Arrange the corned beef on top of the mustard. Spread an even layer of cabbage over the corned beef. Top with the cheese, then the remaining bread slices, buttered side up.
Put the sandwiches on the press, pull the top down, and cook until they are browned and crisp and the corned beef is warmed thorugh, 5 to 8 minutes, depending on how hot your machine is. Carefully remove from the press and serve immediately.
Serve with a Radish & Parsley Salad with Lemon.
Photo: Ellen Silverman