Makes 6 servings.

1 pound flank or sirloin steak
2 tablespoons vegetable oil
1 medium onion, sliced
2 medium portobello mushrooms, trimmed and sliced thin
1 teaspoon salt
1 teaspoon dried oregano
6 long sandwich buns (about 6 inches)
4 to 8 ounces mozzarella or other cheese, sliced
Pickled jalape? slices or spicy pickled okra (optional)

Put the steak in the freezer for 30 minutes to make it easier to slice.

Heat vegetable oil in a wide skillet over medium-high heat. Add the onion and cook, stirring occasionally, until limp and well-browned. This might take 20 minutes.

Meanwhile, slice steak as thinly as you can against the grain. (The grain on flank steak is easy to see - it's the long, striated indentations.) If you end up shaving some of the meat rather than actually slicing it, that's fine. When the onions are brown, increase heat to high and add mushrooms, salt and oregano. Cook, stirring, 6 minutes, until mushrooms are limp and dark.

Add beef and stir quickly and constantly until it's no longer pink.

Divide mixture among buns and top with sliced cheese.

Serve with pickled jalape? peppers or okra, if desired. Serve with a salad of chopped tomatoes and cucumber.