Put the oil, lemon juice, soy sauce, mustard, and garlic in a small bowl, glass measuring cup, or mug; season with salt and pepper. Process with an immersion blender until the garlic is minced and the mixture is thick and smooth, 15 to 20 seconds. Taste and adjust the seasoning.
Put the lettuce in a salad bowl, drizzle with the vinaigrette, and toss to coat. Serve immediately.
