Heat the oven to 400°F.

• Pat the sausages dry with a paper towel; prick in a few places with a fork.
• Peel and thinly slice enough onion to measure 1 cup [2 cups].

In a large frying pan over medium-high heat, warm 2 to 3 teaspoons oil until hot but not smoking. Add the sausages and cook, turning once, until browned and cooked through, 5 to 6 minutes per side. Halfway through the cooking time, move the sausages to one side of the pan. Add the onion to the empty side of the pan, season with salt and pepper, and cook, stirring occasionally, until starting to soften. Transfer the onion to a plate and the sausages to a cutting board. Let the sausages rest for 5 minutes, then cut into ¼-inch-thick rounds.
While the sausages and onion cook, prepare the mozzarella and the flatbreads.

• Tear the mozzarella into 1-inch pieces.

On a sheet pan [using 2 sheet pans if needed], place the flatbreads side by side and brush the tops lightly with oil. Spread the harissa evenly over the flatbreads, leaving a ½-inch border around the edges. Top each with the mozzarella, sausages, and onion. Bake until crisp, 8 to 10 minutes. Transfer to a cutting board and cut into wedges.
While the flatbreads cook, prepare the garlic yogurt.

• Strip the mint leaves from the stems; coarsely chop the leaves. Set aside half for garnish.

In a small bowl, stir together the garlic yogurt and half the mint. Season to taste with salt and pepper.

Transfer the sausage flatbreads to individual plates, garnish with the remaining mint, and serve with the yogurt sauce.