In a bowl combine the steak, soy sauce, olive oil, garlic, and pepper. Let marinate 15 minutes. Warm a nonstick skillet over moderately high heat until it is hot. Add the steak, marinade, and salt, and cook, stirring, just until the meat is no longer pink (be careful not to overcook). With a slotted spoon transfer the steak to a plate. Add the remaining oil to the skillet and warm it over moderately high heat until hot. Add the mushrooms and salt and pepper, and cook, stirring, for 5 minutes.
In a salad bowl combine the steak, mushrooms along with any pan juices, and spinach leaves, tossing gently to combine.
For the dressing: In a bowl whisk together the vinegar, mustard, lemon pepper, and salt to taste. Add the oil, in a stream, whisking until combined well. Whisk in the herbs. Pour the dressing over the salad and toss to combine. Serve with grilled or toasted bread.