Preheat oven to 375°. Coat baking sheet with parchment paper. Set the portobellos gill side up the baking sheet and drizzle with 1 teaspoon of oil. Toss the mushroom mixture and garlic with 1 more teaspoon of oil and spread them out on the baking sheet with the portobellos. Roast for 25 minutes or until tender. Thinly slice the portobellos. Leave the oven on.
While the mushrooms are roasting, sauté the onions in the remaining olive oil over medium heat with a sprinkle of salt and cook for about 20 minutes, until golden brown and caramelized. Stir them often to prevent sticking
In a 9″ x 13″ baking dish, mix together the onions, mushrooms, beans and parsley. Top with the potato mixture. Bake for about 40 minutes, until the potato topping is golden brown.