1. Arrange tofu on several layers of paper towels; cover with additional paper towels. Let stand 30 minutes, pressing down occasionally.
2. Combine lettuce and next 4 ingredients (through onion) in a large bowl. Combine juice and next 4 ingredients (through serrano) in a small bowl, stirring with a whisk. Drizzle half of the dressing over lettuce mixture; toss to coat.
3. Heat a large skillet over medium-high heat. Add peanut oil to pan; swirl to coat. Add tofu and beef; sprinkle with salt and black pepper. Cook 3 minutes or until tofu and meat are lightly browned, stirring once or twice. Remove pan from heat. Add remaining dressing to pan, scraping pan to loosen browned bits. Add tofu mixture to lettuce mixture; toss to combine.