Finely dice the tofu. Rinse, trim, and thinly slice the lemon grass. Rinse and trim the chili. Halve lengthwise, and finely chop. Peel and finely dice the garlic and ginger. Mix the vinegar with the lime juice, sugar, oil, and soy sauce. Mix the dressing with the prepared ingredients and let marinate for 30 minutes.
Peel the cucumber, quarter lengthwise, and julienne. Rinse, trim, and thinly slice the scallions. Mix the cucumber, scallions, and sesame seeds into the tofu mixture. Season to taste.
Meanwhile, peel the cucumber lengthwise into quarters and slice thinly. Wash, clean and cut into rings the spring onions. Both with the sesame quantities under the tofu, season and serve.
Garnish with the mint, and serve.