Preheat the oven to 200°C. Line a muffin tin with paper cases.
In a large mixing bowl stir together the flour, oats, baking powder, baking soda, poppy seeds and sugar.
In another bowl beat the egg lightly, add the oil and milk. Pour this mixture into the flour and combine. Stir in the grated lemon zest and lemon juice.
Fill the muffin case two-thirds full with the batter.
Crumble the butter with the oat & sugar. Top the muffins with the crumble, pressing down gently. Bake in the oven for about 20 minutes until golden brown.
Cool on a wire rack.