Preheat oven to 425 degrees. Scrub potatoes; pat dry. Pierce several times with a fork. Bake until tender, about 1 hour.
While potatoes are baking melt butter in large nonstick skillet over medium heat. Add the green pepper and onion; saute until crisp tender, about 5 minutes. Stir in mushrooms and ham; saute until mushrooms are tender, about 5 minutes. Set aside.
Remove potatoes from oven; reduce oven temp to 350 degrees. Slice tops off of potatoes. Scoop out the insides, keep shells intact. In medium bowl mash potatoes. Stir in 1/2 cup cheddar, milk, and vegetable mixture.
Spoon potato mixture into shells, dividing evenly. Sprinkle remaining cheddar over tops.
Place potatoes on a baking sheet. Bake until hot, about 20 minutes. Serve immediately.
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