Onion Naan

Heat milk in a small saucepan over medium-low heat until an instant-read thermometer registers 100°F. Transfer to a small bowl and whisk in yeast and sugar. Let stand until foamy, about 10 minutes.
Whisk 3 1/2 cups flour and 1 teaspoon salt in a large bowl to blend. Add yeast mixture, onion, yogurt, and 2 tablespoons ghee. Mix dough until blended but still shaggy.
Transfer dough to a lightly floured work surface. Knead until a smooth dough forms, adding flour as needed (dough will be sticky), about 5 minutes. Lightly grease another large bowl with ghee, place dough in bowl, and turn to coat. Cover with plastic wrap. Let rise in a warm, draft-free area until doubled in size, about 1 hour.
Punch down dough and divide into 10 pieces. Using floured hands, roll each piece into a ball on a lightly floured surface. Cover with plastic wrap; let rest 10 minutes.
Heat a large cast-iron or other heavy skillet over medium-high heat. Lightly coat with ghee. Working with 1 piece at a time, stretch dough with your hands or roll out with a rolling pin to 1/8" thickness. Sprinkle with salt. Cook until lightly blistered, puffed, and cooked through, about 2 minutes per side. Wrap in foil to keep warm until ready to serve.
DO AHEAD: Naan dough can be made 4 hours before shaping. Cover and chill.