Roasting brings out sweetness and depth of flavor in the zucchini, onion and garlic. And if warm soups in the summer aren't your thing (I live in Florida, and for some reason am more than okay with soup all year round) this freezes and reheats easily since there's no dairy in it. —wenderzz
Heat oven to 425. Peel onions, cut into wedges with stem end trimmed but still intact; toss onion and garlic clove with 2 teaspoons of olive oil and spread out on a rimmed baking sheet. Season with salt, place in oven.
Cut zucchini into 1/2 inch medallions, toss with 2 teaspoons olive oil and season with salt. After the onions have roasted 30 minutes, flip the onions a bit and add zucchini, making sure it’s spread out and in one layer. Roast for 30-40 minutes more, until onion is soft and golden and edges of zucchini are browned and interiors are soft.
Meanwhile, in a deep pot, heat all but 1/2 cup (reserve to the side) vegetable stock over medium high heat and bring to a simmer. Remove vegetables from oven, squeeze roasted garlic out of skin, and add all veggies to stock. Reduce heat but keep warm.
Place roasting sheet over a burner (or two, depending on size), and heat on medium high. Use reserved stock to deglaze the baking sheet, using a wooden spoon to scrape up as much as you can. Pour deglazed drippings and stock into soup. You can skip this step and just add the reserved stock to the soup if there aren't enough good brown bits on your pan to warrant it. Wipe down your baking sheet for making your croutons.
Add in the vinegar. Using an immersion blender puree until smooth. Taste soup for seasoning; add salt and white pepper to taste. Keep soup warm while you make your croutons.
Set oven to high broil. Cut bread into 1/2 inch cubes and toss with 1 tablespoon olive oil. Spread out on baking sheet, sprinkle with salt and pepper; broil for just a minute or two, until crispy (keep close watch, this goes fast).
Serve soup drizzled with a little extra olive oil and scattered with croutons.