In a bowl, mix the vinegar, oil, onion, sugar, salt, hot sauce, and pepper. Cover and refrigerate 1 hour.
In a skillet over medium heat, cook the walnuts, stirring constantly, until lightly toasted.
In a large bowl, mix the walnuts, romaine lettuce, butter lettuce, spinach, and pears. Toss with the dressing mixture to coat. Sprinkle with feta cheese to serve.