Preheat oven to 300 degrees.
Finely chop the onions. Heat two tablespoons of the olive oil on medium in a 3-quart dutch oven, or 3-quart pot that is ovenproof.
Add the onion and garlic and cook about 8 minutes until translucent and soft. Push the garlic and onion over to one side of the pot and turn the heat up to medium-high.
Add all of the chopped mushrooms and allow them to cook for 5 minutes without stirring.
Turn the mushrooms over and allow them to cook for another 5 minutes. Add the rosemary sprig to the pot and stir the mushrooms and onions together.
Saute for a few minutes. Add the rice and cook for about 4 minutes until the rice begins to turn transparent, stirring only once or twice in the process.
Increase the heat to high and add the wine, vinegar, and vegetable broth, stirring as you add to remove any bits of onion, garlic, or mushrooms sticking to the sides/bottom of the pot.
Add the salt and pepper and stir to combine. Bring to a boil.
Remove from heat and securely cover with a lid and place into the oven for 35 minutes. While the risotto is in the oven, prepare the caramelized onions.
In a large skillet, heat the remaining olive oil over low. Cut the remaining 3 onions in half and thinly slice. Add them to the skillet and generously sprinkle with salt.
Continue to cook on low for 30 minutes until the onions are dark brown, stirring occasionally. Remove the risotto from the oven and let stand for 5 minutes before serving.
Serve with a dollop of Greek yogurt or sour cream and top with the caramelized onions.
Compliments of 365 Days of Baking and More