Wash potatoes; bake at 400° for 1 hour or until done. Let cool to touch. Cut a 1-inch lengthwise strip from top of each potato; carefully scoop out pulp, leaving shells intact. Mash pulp in a large bowl, and set aside.
Combine broth, onion, and garlic in a small saucepan. Bring to a boil. Reduce heat, and simmer, uncovered, until all liquid is evaporated.
Combine potato pulp, onion mixture, sour cream, milk, and pepper; stir well. Stir in Cheddar cheese and 1/4 cup Parmesan cheese. Spoon potato mixture evenly into shells.
Combine remaining 1 tablespoon Parmesan cheese, breadcrumbs, and remaining ingredients; stir well. Sprinkle potatoes evenly with breadcrumb mixture and bake, uncovered, at 375° for 20 minutes or until thoroughly heated.