Preheat oven to 400°F.
Scrub potatoes and dry well. Wipe mushrooms clean with a damp paper towel. Set aside.
Sauté onion and garlic in two tablespoons of olive oil until fragrant. Add portobello mushrooms and continue to sauté for five more minutes. Season with salt and pepper.
Use an apple corer to remove the center of each raw potato. Cut off the ends of the cored potatoes and reserve. Stuff the cored potatoes densely with sautéed portobello mushrooms. Use the reserved ends as caps to close the potatoes. You may have some leftover sautéed mushrooms.
Rub potatoes with the remaining tablespoon of olive oil.
Arrange the stuffed potatoes in a 9×13-inch baking pan. Bake uncovered for one hour. (You may rub the cored-out potato rods with some olive oil and roast them along with the stuffed potatoes.) Slice each potato in half on the diagonal, and serve.