Heat olive oil over medium heat. Add sliced onions and stir. Season with a pinch of sea salt. Reduce heat to medium-low and continue cooking, stirring every few minutes until golden brown, for about 30 minutes.
Once caramelized, add vegetable stock to deglaze the pan. Add mushrooms and garlic and season to taste with sea salt and pepper. Cook until mushrooms are browned and tender, about 6 minutes. Simmer on low for 5 minutes, stirring often.
Sear polenta rounds in 1 Tbsp. olive oil and season each side with freshly cracked pepper. Cook polenta until golden brown on both sides, about 5 minutes per side.
Top polenta with mushroom mixture and fresh parsley. Serve warm.