Follow these simple steps to create your One-Bowl Banana Bread with Crunchy Streusel Topping:

1. Preheat the Oven: Preheat your oven to 350°F (175°C) and grease a 9×5-inch loaf pan.
2. Prepare Bananas: In a large mixing bowl, mash the ripe bananas with a fork until smooth.
3. Combine Wet Ingredients: Add the melted butter to the mashed bananas and mix well. Then, stir in the beaten egg and vanilla extract.
4. Mix in Sugar: Add the granulated sugar and mix until fully incorporated.
5. Combine Dry Ingredients: Sprinkle the baking soda and salt over the mixture and stir to combine.
6. Add Flour: Gradually add the flour, mixing until just combined. Avoid overmixing for a fluffy texture.
7. Prepare the Streusel Topping: In a separate bowl, mix the flour, brown sugar, oats, cinnamon, and cold butter. Use a fork or your fingers to combine until the mixture resembles coarse crumbs.
8. Assemble: Pour the banana bread batter into the prepared loaf pan. Evenly distribute the streusel topping over the batter.
9. Bake: Place the loaf pan in the preheated oven and bake for 60-65 minutes. The bread is done when a toothpick inserted into the center comes out clean.
10. Cool: Once baked, remove the banana bread from the oven and let it cool in the pan for about 10 minutes. Then transfer it to a wire rack to cool completely.

By following these steps, you'll create an incredible loaf of banana bread that will impress everyone!