Preheat oven to 375°F. Rub potatoes with 1 Tbsp. oil; pierce with a knife. Bake until tender, 1 hour and 30 minutes. Steam broccoli until tender, 5 minutes. Drain; rinse. Pat dry and roughly chop.
In a skillet over low heat, warm 1 Tbsp. oil. Sauté onion until soft, 10 minutes. Add garlic; cook 2 minutes. Remove from heat.
Let potatoes rest until cool enough to handle. Set oven to 350°F. Cut top 1/4 inch off potato. Scoop out flesh.
Mash potato flesh. Mix with remaining ingredients. Fill potato shells with mixture; bake 30 minutes.