Whisk the vinegar, mustard, salt, and pepper together in a bowl. (see note)
Gradually drizzle in the olive oil, about a teaspoon at a time, whisking all the time, to make a smooth dressing.
Wash the lettuce leaves, dry well. Tear into bite-size pieces.
Just before serving, put about half of the vinaigrette in a large bowl. Add the lettuce and toss to coat the leaves lightly. Toss in a little more dressing if you think it’s needed, keeping in mind the lettuce should be lightly coated, not weighed down in oil.
Build the salad in two layers: Transfer half of the dressed lettuce to a serving bowl. Core and slice the first pear and arrange it over the lettuce. Sprinkle with half of the blue cheese and half of the toasted pecans. Repeat with a second layer.