Soak the porcini mushrooms in 1 cup warm water for 5-10 minutes.
Heat a large pot or Dutch oven on medium high. When hot, add olive oil, along with the onions, carrots, celery. Add the sugar and season with salt and black pepper. Cook, stirring occasionally, until the vegetables have softened and are starting to turn golden, 7-8 minutes.
Meanwhile, remove the porcini mushrooms from the soaking water (keep the water!), squeeze out the liquid and roughly chop the mushrooms. Add the vegetable stock to the porcini soaking water - you should have about 2 cups of stock now.
To the vegetables, add the garlic, rosemary and sage and stir for 1 minute until fragrant. Add the wine and cook for one minute, stirring, to allow the alcohol to cook off. Add the porcini mushrooms and then the other mushrooms, season with salt and pepper, and cook, stirring occasionally, until the mushrooms have started to soften, about 2 minutes.
With the heat still on medium high, add about 1/2 cup of stock and cook, stirring occasionally, for 2 minutes. Add another 1/2 cup of stock, and cook, stirring occasionally, for another 2 minutes. Repeat this process 2 more times until the stock has been used up.
After all the stock has been added, cover with lid, reduce heat to medium-low and cook for 5 minutes. Turn off the heat. Uncover and stir in the butter and vinegar. Taste and season with salt and pepper if needed.
Serve the mushroom ragu bianco with pasta, rice or polenta.