Preheat the oven to 350°F (180°C, or gas mark 4).

Carefully remove the stems from the mushrooms, reserving the stems for the filling.

Combine balsamic vinegar and olive oil in a shallow baking dish to prepare the marinade.

Place mushrooms in the dish, "hole" side up.

Set aside.

Chop the reserved mushroom stems finely.

Heat olive oil in a sauté pan over medium heat.

Add chopped mushroom stems, onion, and garlic.

Sauté until fragrant, about 7–10 minutes.

In a small pot, bring water to a boil.

Add couscous, reduce to a simmer, and stir until the water is fully absorbed.

Mix the couscous with the sautéed vegetable mixture, curry powder, parsley, garam masala, salt, and pepper until well combined.

Fill each mushroom with a heaping scoop of the couscous mixture.

Bake the mushrooms, uncovered, in the oven for about 15 minutes.

Serve warm.

Any leftover couscous can be enjoyed as a flavorful side dish.