In a stockpot over medium-high heat, add oil and swirl to coat the bottom. Add jalapeño, ginger, garlic and scallions. Sauté until aromatic, about a minute. Add shiitakes and sauté 2 minutes, until softened. Add stock and soy sauce, bring to a simmer and reduce by 20%, about 5 minutes. Add lemon juice, tofu and carrots and cook gently for 2 more minutes to heat tofu through. Season with black pepper. Ladle soup into serving bowls and garnish with reserved scallion greens and lemon zest.