Preheat the oven to gas mark 4/350oF/180oC/160oC fan and line a 12-count muffin tin with paper muffin cases.
Make the crumble topping: Add the plain flour and oats to a bowl then add in the butter. Use your fingers and coat the butter cubes with the flour and oats then use a folk and break up the butter into the flour and oats. Once the butter has broken up, add in the granulated sugar, salt and cinnamon then mix them together while breaking up any remaining pieces of butter. Set the bowl aside.
Cut the pears and remove the cores then remove the skins. Chop the pears into small cubes (approximately ½ inch pieces) then add them to a bowl and set aside.
Make the muffin batter: Add the butter and caster to a bowl then cream them together until well combined. Pour in the vegetable oil and continue to mix until smooth then mix in the eggs. Mix in the vanilla extract and Greek yoghurt then add in the salt and self-raising flour. Lastly, fold in the chopped pears until combined. The batter will be quite thick.
Spoon the batter into the 12 muffin cases then top the muffins evenly with the crumble topping. Gently press the crumble topping into the muffin to ensure the topping doesn’t fall off when the muffins rise.
Bake the muffins for 25 minutes until golden. You can test the muffins with a cake tester or skewer to ensure they are baked through. Place the muffin tin on a cooling rack for 20 minutes then remove the muffins from the tin and place them directly onto the cooling rack to completely cool.