Preheat oven to 450ยบ F.
Line a rimmed baking sheet with parchment paper, toss romanesco, onions, and mushrooms, 2 tablespoons olive oil, and 1/2 teaspoon salt. Roast until brown, stirring occasionally for 30-35 minutes.
Meanwhile in a large pot, bring 6 cups of water and 1 teaspoon of salt to a boil. Slowly whisk in the polenta. Turn heat down to a simmer, stirring often until polenta has thickened, about 5-10 minutes.
In a mini food processor, pulse 2 tablespoons olive oil, parsley, thyme, pine nuts, capers, lemon zest, lemon juice until parsley is minced. Add to roasted vegetables and mix.
Stir the butter and parmesan into the polenta.
To serve; spoon polenta into a wide shallow bowl and top with roasted vegetables.