Toast coriander seeds in small skillet over medium heat until aromatic, about 2 minutes. Transfer to mortar and grind coarsely with pestle. Transfer to small bowl. Mix in lemon juice and shallot. Gradually whisk in olive oil. Season dressing to taste with salt and pepper.
Heat heavy medium skillet over medium-high heat. Add pancetta and sauté until crisp, about 4 minutes. Transfer to paper-towel-lined plate. Place lettuce in large bowl. (Dressing, pancetta, and lettuce can be prepared 2 hours ahead. Let dressing and pancetta stand at room temperature. Cover lettuce with damp kitchen towel and refrigerate.)
Toss lettuce with dressing. Top with pear slices, then walnuts and pancetta.