In a medium size saucepan, gently sweat the onions and garlic in olive oil over medium heat.

Once the onions are translucent and tender, add the tomatoes and a sprig of basil.

Continue to cook on medium heat until the tomatoes are soft and completely cooked through, then remove from the heat.

Using tongs, remove the basil from the pan.

Transfer the soup base to a blender and blend on high until smooth.

Pass the contents of the blender through a fine mesh strainer into a large bowl.

Add the balsamic vinegar.

While whisking vigorously, drizzle in olive oil until the soup takes on a shiny finish.

Season with kosher salt to taste.