Preheat a cast-iron grill pan or griddle over medium-high heat.
Slice the sausages lengthwise and open them like a book. Remove the casings and press the sausages until flattened to about 1/4 inch thick. Combine the cumin and oregano in a small bowl. Rub the cut-side of the sausages with the spice mixture. Brush the hot pan with oil and add the sausages, seasoned-side down, and cook each side until the sausages are browned and cooked through, 3 to 4 minutes per side.
Toss the tomatoes and their juice with the lettuce in a medium bowl. Stir the yogurt, lemon juice, harissa and 1/4 teaspoon salt together in another bowl until smooth.
Warm each flatbread in a dry skillet for about 30 seconds per side. Spread some of the yogurt sauce on the bread and then top with the tomato-lettuce mixture and sausage. Serve the extra sauce on the side.
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