Method:
Preheat oven to 400°F. Arrange potatoes on a rimmed baking sheet or in a muffin tin and bake, flipping halfway through, until just tender, 35 to 40 minutes. Carefully halve potatoes, arrange on baking sheet; cool briefly for handling. Using a teaspoon, scoop out the middle of each potato half, leaving a 1/4-inch border around the edges and bottom. Transfer potato flesh to a large bowl and mash roughly with a fork. Add 2/3 cup cheddar, and the ham, yogurt, butter, salt, pepper and onions. Stir well. Scoop some of the potato mixture into each potato half. Scatter remaining 1/3 cup cheddar over the tops. Bake until golden brown and hot throughout, about 15 minutes more.