Pear, Avocado and Mixed Greens with Pistachio and Tarragon Dressing

1 firm but ripe avocado
1 firm but ripe pear
2-3 handfuls mixed green salad leaves (I used cos and buttercrunch lettuces)
2 tablespoons pistachio nuts

Dressing
3 tablespoons olive oil
1 tablespoon chopped pistachio nuts
1 clove garlic, crushed
1 tablespoon lemon juice or white wine vinegar
½ teaspoon dried tarragon
sea salt and freshly ground pepper