Preheat oven to 400°F. Line a muffin tin with muffin liners or lightly grease with cooking spray.
In a large bowl, whisk together the egg, oil, and brown sugar for about 1 minute until slightly lighter and combined.
1 Lrg Egg, ¼ Cup Neutral Oil, ½ Cup Brown Sugar
Add the milk and stir until smooth.
1 Cup Milk
Add flour, baking powder, salt, cinnamon, nutmeg, and oats directly into the same bowl. Stir just until combined. Do not overmix.
1 Cup All-purpose flour, 2 Tsp Baking powder, ½ Tsp Salt, ½ Tsp Cinnamon, ¼ Tsp Nutmeg, ¾ Cup Oats
Fold in the grated carrots until evenly distributed.
¾ Cup Carrots
In a small bowl, mix the streusel topping ingredients until crumbly.
Fill muffin cups. For regular muffins, fill to the top. For jumbo muffins, fill about ⅔ full.
Sprinkle streusel evenly over each muffin and gently press it into the batter.
⅔ Cup All-purpose flour, ½ Cup Brown sugar, ¼ Cup Butter, ½ Tsp Cinnamon
Bake for 20–25 minutes, or until golden and a toothpick inserted into the center comes out clean.
Let cool in the pan for 5–10 minutes before removing. Muffins will be soft at first and firm up as they cool.