In a large sauté pan, heat the oil over medium heat. Add the onion and jalapeno and cook until onion is tender and translucent, about 3 minutes.
Add tomatoes and cook for another 2 minutes, or until tomatoes just start to lose their shape.
Add the greens and a sprinkle of sea salt and black pepper. Cook, stirring frequently, until greens are wilted and just tender. Sturdier greens will take longer than baby greens.
Pour lemon juice on top and taste for seasoning. Serve immediately.