Preheat a grill to a moderate heat, then toast the baguette slices lightly on both sides.
Cut the avocados in half. Remove the stones, peel off the skins, then thinly slice the flesh. Sprinkle with half the lemon juice and season with salt. Peel, quarter and core the pears. Cut the flesh into slices and sprinkle with the remaining lemon juice. Put the salad leaves in a bowl.
To make the dressing, put the oils, vinegar, mustard and seasoning in a screw-top jar and shake vigorously. Pour over the salad leaves and toss well.
Place a round of cheese on each slice of baguette and heat for 2–3 minutes under the grill until golden on the top and just beginning to melt.
Divide the salad leaves among four plates and scatter with the slices of avocado and pear. Place a cheesy baguette croûte on each serving, sprinkle with pepper and serve.