Make the crumb topping: Combine the flour, brown sugar, cinnamon, butter, and salt in a small bowl. Chill until firm, at least 30 minutes.
Roast the strawberries: Heat oven to 375° F. Toss the strawberries and granulated sugar on a rimmed baking sheet. Roast, tossing once, until very soft and broken down, 20 to 25 minutes. Let cool.
Make the muffins: Butter and flour a standard 12-cup muffin tin. Whisk together the all-purpose flour, whole-wheat flour, granulated sugar, flaxseed meal, baking powder, cinnamon, salt, and baking soda in a large bowl. In a separate bowl, whisk together the milk, eggs, vanilla, and melted butter. Add the wet ingredients to the dry ingredients and whisk to combine. Fold in the roasted strawberries.
Divide the batter between the prepared muffin cups. Top with the chilled crumb topping.
Bake, rotating once, until a toothpick inserted in the center of a muffin comes out clean, 25 to 30 minutes. Cool in the pan for 5 minutes. Transfer to a wire rack to cool completely.