Yogurt-Peach Semifreddo

Line 8 1/2x4 1/2" loaf pan with plastic wrap, leaving a generous overhang on all sides. Cook peaches and 1/4 cup sugar in a large skillet over medium heat, stirring occasionally, until peaches are softened and sugar is dissolved, about 5 minutes. Transfer to a blender and purée until smooth. Let cool. Meanwhile, whisk egg whites, salt, and remaining 1 cup sugar in a medium heatproof bowl (or the bowl of a stand mixer) set over a saucepan of simmering water (do not let bowl touch water). Cook, whisking constantly, until sugar is dissolved and mixture is warm to the touch, 2 –4 minutes. Remove bowl from saucepan. Using an electric mixer on high speed, beat egg mixture until tripled in volume, glossy, and completely cool, about 10 minutes. Whisk yogurt and lemon juice in a large bowl, then stir in mint. Gently fold in egg white mixture until combined. Transfer half of the yogurt mixture to prepared pan; smooth surface. Swirl half of the peach mixture into yogurt layer with a spoon, then repeat with remaining yogurt and peach mixtures. Fold plastic wrap overhang over top to seal and freeze until firm, at least 8 hours. Unwrap semifreddo and, using plastic overhang, gently lift from pan. Invert semifreddo onto a cutting board, remove plastic wrap, and let sit at least 10 minutes to soften. Slice into 6 even slices, about 1 1/2" thick; transfer to plates and serve.