Peel and halve the red onions and slice them thinly crosswise. Heat 2 tablespoons olive oil in a large nonstick pan and saute the onions with a pinch of salt, stirring often on medium heat until they are soft and lightly browned in sports, about 30 minutes - and don't hurry this.
Meanwhile, scrub and trim the potatoes and cut them into 1/2 inch dice. In a large soup pot, combine the potatoes and whole garlic cloves with enough water to cover (about 2 cups) and a scant teaspoon of salt. Simmer the potatoes and garlic for about 10 minutes, or until tender but not falling apart.
Peel the tomatoes, either by blanching and slipping off the skins or by using a serrated peeler, and cut them in pieces, saving all the juice. Add the caramelized onions to the potatoes and garlic; along with the tomatoes and their juice, the wine and all the herbs.
Simmer the soup, covered, until all the veggies are soft, about 30 minutes. Add the veggie broth, the amount you need will vary on the juiciness of the tomatoes. The soup should pour easily from the ladle, the broth slightly thickened by the potatoes and perfumed with the sage and garlic.
Taste the soup and correct the seasoning with more salt and freshly black pepper. Drizzle with a little extra olive oil, if desired.