Preheat the oven to 200°C fan forced. Line a 12-hole (⅓ cup-capacity) muffin pan with paper cases.
To make the oat crumble topping; combine the oats, flour, nutmeg and sugar in a small bowl. Add the butter and stir until crumbly. Squeeze together with your fingertips to form crumble. Stir in the grapes.
Sift the flour into a large bowl. Add the nutmeg and brown sugar and stir until combined.
Whisk together the milk, eggs, butter and vanilla until combined. Add to the flour mixture and stir gently until almost combined. Gently stir in the grapes, do not overmix. Spoon evenly among the paper cases.
Top with crumble mixture, pressing evenly over the top. Bake for 25 minutes or until golden and cooked through when a skewer inserted in the centre comes out clean. Stand the muffins for 10 minutes in the pan. Transfer to wire rack. Serve warm or at room temperature.
TipStore muffins in an airtight container up to 2 days in a cool place or wrap individually in plastic wrap and freeze for up to 3 months.
TipThese grape muffins are perfect for lunchboxes or afterschool treat.