Move rack to top of oven and set broiler to high. Shave just the roots from onions, leaving core intact so that onion layers stay in place. Halve or quarter any larger onions. Arrange on a broiler pan and slide into oven. Broil until edges begin to char and onions are tender but still have some bite, about 10 minutes.
Meanwhile, set a Dutch oven or small stock pot over medium heat. Add olive oil, and when hot, add potatoes, 1/2 teaspoon sea salt, and a few twists pepper. Sauté potatoes until tender with crisp edges, about 7 - 10 minutes. Add white wine, stir 1 minute, and then add broth. Bring mixture to a boil, then turn heat to low. Taste and add sea salt as needed.
Working carefully in batches, blend charred onions, broth and potato mixture, and greens until smooth. (I like this soup with some texture, so I blend until the texture is smooth but still flecked with bits of char.)
Pour back into soup pot and warm over medium-low heat just until mixture starts to steam. Heating soup gently is the best way to maintain its emerald green hue.
Ladle into bowls and garnish with shaved Parmesan, minced onion grass or chives, thinly slice green onions, and several twists white peppeWipe any vinegar spills from the rim with a clean towel and put on the lid.
Hide the jar in the back of the fridge for at least a week. Two weeks is better, three is best.
Keep them in the fridge for up to 6 months.
1 pound spring onions (a mix of spring or salad onions and green onions), greens attached
2 tablespoons extra virgin olive oil
1/2 teaspoon sea salt, plus more to taste
fresh ground white pepper
1 medium Yukon Gold potato chopped into 1/4-inch pieces (1 heaping cup)
1/2 cup dry white wine
1 quart low-sodium vegetable or chicken broth
3 cups young leafy greens (I used a mix of kale and arugula)
1/4 cup shaved Parmesan
4 - 5 chives or blades of onion grass, minced
1 green onion, sliced thin, for garnish