Preheat the oven to 175 degrees Celsius. Line cupcake moulds with cupcake liners.
Measure the plain flour, instant oats, baking powder, baking soda and salt in a medium bowl. Whisk with a wire whisk to mix them altogether and to get rid of any huge lumps.
In a separate bowl or measuring cup, combine the egg, egg yolk, sour cream (or buttermilk), lemon zest (if using), vanilla extract, olive oil (or vegetable oil or melted unsalted butter). Whisk with a wire whisk to mix them altogether. Add in the brown sugar and whisk until the sugar has dissolved (not all will dissolve but that's alright).
Pour the liquid mixture directly into the centre of the dry mixture and whisk together gently until all the dry mixture has been fully incorporated.
Spoon the batter into the cupcake cups until half-full, top with 3 strawberry quarters - make sure the batter does not exceed 2/3 of the cupcake liner, otherwise it will overflow.
Bake for 15 to 20 minutes until a toothpick inserted into the centre of the cake comes out clean. Let the cupcakes cool in the tin for 2-3 minutes then remove them to cool completely on a wire rack (so that condensation doesn't occur on the bottom of the cupcakes).