Bartlett Pear Muffins

• 1 egg (beaten)
• 1 lemon zest
• 250 gr all purpose flour
• 240 ml milk
• 200 gr pears about (diced)
• 100 gr granulated sugar
• 60 gr walnuts (chopped)
• 55 gr melted butter cooled
• 14 gr baking powder
• 1 gr salt