Put the tofu cubes on paper towels and let sit 30 minutes so they give up some of their moisture.
In a salad bowl, combine lettuce, mint, basil, cucumber and onion.
Make the dressing: combine the lime juice, fish sauce, sesame oil, sugar and Thai chilli and whisk until the sugar has dissolved. Drizzle half the dressing over the lettuce and toss.
Heat a heavy frypan with 1 teaspoon oil and add the tofu and beef. Sprinkle with salt and pepper and cook until everything is seared, about 5 minutes. Add the remaining dressing to deglaze and stir around. Add the tofu and beef to the salad and serve.