2 medium yellow onions
2 Tbsp plus 2 tsp extra-virgin olive oil, divided
1/2 tsp salt, divided
2 Tbsp Thai green curry paste (available at any Asian grocery store or gourmet market)
4-5 Cups vegetable broth
8 Cups loosely packed spinach (about 6 oz), stems trimmed, divided
2 Cups water
2 Cups shiitake mushrooms (about 4 oz), trimmed, or 1 oz dried shiitake mushrooms, soaked for 20 min in boiling water
2 cloves garlic, minced
1-1/2 cups 1-inch pieces fresh or froz green beans
1 Cup thinly sliced peeled broccoli stems
5 scallions, sliced
1 Tbsp finely-chopped lemongrass
1 Cup fresh cilantro, coarsely chopped
1 fresh serrano chile, halved, de-veined, and finely chopped (OR 1 tsp Serrano Chile Green Powder) — optional
2 Tbsp fresh lemon juice, or more to taste
Quarter onions lengthwise and remove outer layer, then thinly slice and drop into 2 Tbsp oil in a heavy pot, heated over medium-high heat. Add 1/4 tsp salt and cook, stirring frequently, until the onions become soft and a little translucent, about 8 min. Stir in green curry paste and continue to cook, stirring, for another 3 min. Gently pour in 4 cups of the vegetable broth, stirring to combine curry paste and liquid. Bring to a gentle simmer.
Meanwhile, coarsely chop 4 Cups of the spinach. Place the remaining 4 Cups of spinach with water in a blender; puree mixture until the spinach is chopped to confetti (but not liquefied).
Rehydrate mushrooms (if needed). Cut shiitake mushrooms into 1/4-inch strips.
Heat the other 2 tsp oil in a large heavy skillet, over medium heat. Add the minced garlic and cook, stirring constantly until garlic becomes fragrant, only about one minute. Add the mushrooms to the garlic and cook, stirring to combine, until any liquid evaporates and the mushrooms begin to brown lightly, about 6 min.
Transfer the garlic-mushroom mixture and the green beans into the soup pot; return pot to a simmer and allow to bubble gently for 5 minutes. Stir in broccoli stems, chopped scallions and lemongrass; return soup to a simmer and cook for 3 minutes more.
Add to the pot the chopped and pureed spinach, cilantro and, if desired, a big pinch of serrano chile. Return soup to a simmer, then cover the pan with its lid, and cook just long enough to wilt the spinach, barely 1 minute.
Add up to 1 Cup additional broth if a thinner consistency is desired. Just before serving, add fresh lemon juice; taste and add more salt, serrano and/or lemon juice, if desired.