Preheat the oven to 350F / 170C.

Carefully remove the stems from the portobellos, using a small knife. Chop the stems and set aside. Using a teaspoon, scrape the inside of the caps clean, careful not to break the rims or mushrooms in any way.

Rub the portobellos with 1 – 2 tablespoons of olive oil. Season with salt and pepper and sprinkle with a half tablespoon each of the balsamic.

Drizzle 2 tablespoons of the olive oil in a small ceramic or ovenproof glass baking dish large enough to hold the mushrooms snugly. Strew the garlic cloves in the center and place the mushrooms on top. Fill each mushroom with 3 tablespoons of the fava. Cover with parchment and then aluminum foil and bake for about 12 – 15 minutes, until the mushrooms are tender but al dente and the fava set.

While the mushrooms are baking, heat 2 tablespoons of olive oil in a medium, heavy skillet. Add the onions, season with a generous pinch of salt and cook until wilted and lightly golden, about 12 – 15 minutes. Add the chopped mushroom stems a few minutes before removing from heat. Toss in the chopped parsley. Season to taste with salt and pepper.

Serve the stuffed mushrooms topped with the onion mixture and drizzle with remaining olive oil and additional balsamic if desired.