1. Pierce potatoes liberally with a fork. Microwave at HIGH 14 minutes or until tender. Remove potatoes from microwave, and cool for 10 minutes.
2. While potatoes cook, bring 2 tablespoons water to a simmer in a large skillet over medium-high heat. Add spinach to pan; cook for 2 minutes, stirring until spinach wilts. Cool for 5 minutes. Place spinach in a paper towel, and squeeze out any excess liquid. Coarsely chop spinach.
3. Cut one-third off each potato lengthwise. Remove pulp from potato, leaving a 1/8-inch-thick shell. Combine potato pulp, yogurt, salt, pepper, ham, cheese, and spinach in a large bowl, stirring to combine. Evenly fill potato shells with spinach mixture; sprinkle with green onions.