Preheat oven to 375º. Line the cups of a muffin pan with paper liners (or grease them) and set aside.
In a large bowl, whisk together flour, granulated sugar, baking powder, and salt.
In a medium-size bowl, stir together egg, milk, oil, vanilla, and cinnamon.
Add wet ingredients to dry ingredients and stir with a fork until just moistened.
Fold in rhubarb and strawberries.
Fill muffin cups two-thirds full.
Combine brown sugar and cinnamon and sprinkle over batter; gently place a strawberry slice on top of each muffin.
Bake until golden brown, 20–25 minutes. Remove from tin and cool on a wire rack.