In a big bowl, whisk together flour, sugar, baking powder, eggs, oil and vanilla and mix together until well combined. Add bananas to the egg mixture and stir until just moistened.
Mix the strawberries with 1 tbsp all-purpose flour to ensure that the strawberries don’t sink to the bottom of your muffins and fold into the batter until combined.
For the crumble: combine flour and sugar in a bowl. Mix in butter with fingertips until moistened and crumbly.
Scatter over muffins and bake until golden and just set, about 20-25 minutes. Cool completely on a rack.