In blender container or food processor bowl with metal blade, combine sugar, salt, mustard, vinegar and onion. Cover; blend until combined. With machine running, add oil in a slow, steady stream, blending until thick and smooth. Add poppy seed; blend a few seconds to mix. Refrigerate until ready to serve.
Sprinkle lemon juice over peach slices; reserve 1/2 cup for garnish. In large serving bowl, combine remaining peaches, lettuce and spinach; toss gently. Arrange reserved peach slices over greens mixture. Sprinkle with almonds. Serve dressing with salad.