Preheat and Prepare Strawberries - Preheat your oven to 400°F (204°C) and line a muffin tin with paper liners (spray the inside of the paper liners with oil) or lightly grease the cups.
Prepare the Strawberries - Wash the strawberries, cut off the stems, and chop them into small pieces, then sprinkle them with a tablespoon of lemon juice to preserve their color.
Prepare the Streusel Topping - In a small bowl, combine the all-purpose flour, rolled oats, brown sugar, softened vegan butter, salt, baking soda, and ground cinnamon. Mix everything together until crumbly. Set the streusel topping aside.
Mix the Dry Ingredients - In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt until well combined.
Combine the Wet Ingredients - In a separate bowl, whisk together the plant milk, oil, apple sauce, lemon juice, and vanilla extract.
Bring it All Together - Pour the wet ingredients into the bowl of dry ingredients. Stir gently until just combined. Be careful not to overmix; a few lumps are okay. Fold in 1 1/2 cups chopped strawberries, ensuring they are evenly distributed throughout the batter.
Fill the Muffin Cups - Spoon the muffin batter into the prepared muffin cups, filling each about three-quarters full. Leave some space for extra strawberries and the streusel topping.
Add the Streusel Topping - Add a 1/2 tablespoon of chopped strawberries to the tops of each muffin then Sprinkle the prepared streusel topping generously over each muffin.
Bake and Enjoy - Turn down the heat in the oven to 375° F (190° C) then place the muffin tin in the preheated oven and bake for 18-20 minutes until golden brown, or until a toothpick inserted into the center of a muffin comes out clean. Once baked, remove the muffins from the oven and let them cool in the tin for a few minutes before transferring them to a wire rack to cool completely.
Notes
Be sure to spoon-measure your flour, so you have an accurate measurement. Make sure to preheat your oven to 400° F (200°C) and that it has reached that temperature before putting your muffins in the oven or they will not rise as much.
If using cupcake liners, be sure to spray them will a little oil so that the muffin doesn't stick to the paper.