In a large nonstick skillet, heat 2 tbsp. EVOO over medium-high heat. Add the mushrooms; toss to coat. Cook, stirring occasionally, until tender and golden, about 7 minutes. Add the onion and cook 1 minute. Toss in the parsley; season.
Meanwhile, in a medium saucepan, bring 4 cups water to a boil. Add the polenta and cook, stirring, until thickened, 2 to 3 minutes. Whisk in the gorgonzola; season.
Divide the polenta among four bowls. Top with the mushrooms and watercress; drizzle with some EVOO.