In a large bowl, whisk together the mustard, vinegar, and 2 tablespoons of the oil. Set aside. In a large skillet, lightly toast the walnuts over medium heat, stirring occasionally, 5 to 6 minutes. Remove and set aside. Add the bacon and cook until crispy, 3 to 4 minutes. Remove the bacon and drain on paper towels. Discard all but 1 tablespoon of the fat from the pan. Add the remaining olive oil and the cabbage and toss to coat well. Add 1/4 cup water, the lemon juice, salt, and pepper and cook until soft, about 5 minutes. Remove from skillet and add to the vinaigrette. Add the frisee, walnuts, and bacon (broken into pieces) and toss well.