Makes a dozen muffins.
Preheat the oven to 200C (400F) and grease a 12 cup muffin tin.
Make the streusel topping. In a medium bowl, combine 45 g (½ c) rolled oats, 40 g (⅓ c) plain flour*, 40 g (⅓ c) chopped pecans or walnuts, 66 g (⅓ c packed) light brown sugar, 1 heaped t of ground spices (cinnamon, nutmeg, ginger, etc.), and a pinch of salt. Pour 4 T melted butter over the mixture and mix thoroughly to combine in to a crumb-like mixture.
Make the batter. In a large bowl, thoroughly combine 240 ml (1 c) milk*, 113 g (½ c) cooled melted butter, 1 large egg, 100 g (½ packed c) light or dark brown sugar, and 1 T white vinegar. In a separate bowl, combine 125 g (1 c) plain flour, 90 g (1 c) rolled oats, 1 t ground cinnamon, a pinch of salt, 1 t baking powder, and ½ t baking soda. Stir the dry in to the wet just until combined.
Immediately after making the batter, ladle it in to the prepared muffin tin. The batter should fill the cups about ¾ of the way. Next, add a generous tablespoon of topping to each muffin.
Bake in a 200C (400F) oven for 18–25 minutes. Allow to cool for at least 5 minutes in the tin, then remove the muffins to a rack to continue cooling.
* This seems like a substitution-friendly recipe. I made these with a combination of dark rye flour and gluten free flour when I didn’t have normal flour in for whatever reason and they came out really well. Wouldn’t hesitate to try all GF flour or similar substitutes. Oat milk works really well too.