Heat olive oil in a large pot over medium heat, add the onion, potato, and salt and sauté until ~ 10 minutes. Add the garlic and red pepper flakes and stir for another 30 seconds. Pour in ½ cup of the broth to deglaze the pot and cook until the liquid is reduced by half.

Add in the collards and chard and another ¼ teaspoon of salt. Stir well to combine so that the greens will wilt. Then add the remaining 5½ cups of broth, bring to a boil, reduce the heat to medium, cover, and simmer for ~10 minutes.

In a blender, puree the soup until smooth. Blend the parsley into the last batch (this brightens the color to a gorgeous green). Stir in the lemon zest and juice. Taste and add more lemon and/or salt as needed.