Preheat oven to 375°. Scrub potatoes; pierce several times with a fork. Place in a foil-lined 15x10x1-in. baking pan; bake 60-70 minutes or until tender.
When cool enough to handle, cut each potato lengthwise in half. Scoop out pulp, leaving 1/4-in.-thick shells. In a large bowl, mash pulp with milk and butter, adding spinach, Monterey Jack cheese, 1/4 cup cheddar cheese, onion, salt and pepper.
Spoon into potato shells; return to baking pan. Sprinkle with remaining cheddar cheese. Bake 20-25 minutes or until heated through and cheese is melted.