With a coring tool, carefully scoop out as much of the inside of the squash without piercing through the skin. Save the inside for later use with Zucchini Muffins.
Stuff the meat mixture into the squash. Gently press down and fill to the rim.
Arrange the squash around the bottom of a large pot, stacking them as necessary. Barely cover with water and simmer, partially covered for 30 minutes or until the zucchini is tender when pierced with a fork.
Drain the water and stir in yogurt sauce.