Crumble topping:
scant 3/4 cup / 3 oz / 85 g whole wheat pastry flour
1/2 cup / 1.5 oz / 45 g rolled oats
1/2 cup / 2.5 oz / 70 g natural cane or brown sugar
scant 1/2 teaspoon fine grain sea salt
1/3 cup / 2.5 oz / 70 g unsalted butter, melted

Muffin batter:
1 cup / 3.5 oz / 100 g rolled oats
3/4 cup / 3.5 oz / 100 g unbleached all purpose flour
3/4 cup / 3.5 oz / 100 g whole wheat pastry flour
1/2 teaspoon baking soda
scant 1/2 teaspoon fine grain sea salt
7/8 cup / 7 oz / 200 g unsalted butter, plus more for greasing pan
1/2 cup / 2.5 oz / 70 g natural cane or brown sugar
1 1/2 cups / 12 oz / 350 ml plain yogurt
2 large eggs, whisked