Add onion, chopped chile, ⅓ cup Green Seasoning, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt to pot and cook, stirring occasionally, until onion is softened and translucent, about 3 minutes. Add potatoes and remaining whole chile and reduce heat to medium-low. Cover pot; cook, stirring occasionally and adding a splash or so of water if mixture looks dry or is sticking to bottom of pot, until potatoes are fork-tender, 30–35 minutes.