Combine the oats, brown sugar, cinnamon, and 1 tablespoon melted butter in a small bowl, stirring until the mixture resembles coarse breadcrumbs with some texture remaining. Set aside—this streusel will add a delicious crispy contrast to the tender muffin crumb. I like to make this first so it's ready to sprinkle on top right before baking.

Preheat your oven to 425°F and grease a standard 12-cup muffin tin with butter or cooking spray. While the oven heats, peel and mash the 3 bananas in a bowl until mostly smooth with a few small lumps remaining, then dice the strawberries into ½-inch pieces and toss them lightly with 1 tablespoon flour—this coating helps prevent them from sinking to the bottom during baking.

In a large mixing bowl, beat ½ cup softened butter and ¾ cup sugar together on medium speed for 2-3 minutes until the mixture is pale, fluffy, and increased in volume. This creaming step incorporates air into the batter, which helps the muffins rise and creates a tender crumb structure.

Add the 2 room-temperature eggs one at a time to the creamed butter and sugar, beating well after each addition until fully incorporated. Pour in the vanilla extract and stir in the mashed bananas from Step 2, mixing just until combined. The eggs and bananas add structure and moisture; I find using room-temperature eggs helps them integrate more smoothly without deflating the batter.

In a separate bowl, whisk together the 2 cups flour, 1 teaspoon baking soda, and ½ teaspoon salt. Add the dry mixture to the wet ingredients and stir gently just until barely combined—don't overmix, as this keeps the muffins tender. Gently fold in the flour-coated strawberries from Step 2 using a spatula, being careful not to crush them.

Divide the batter evenly among the 12 muffin cups, filling each about ¾ full. Sprinkle the oat streusel topping from Step 1 generously over each muffin, using it all to create a nice crunchy layer on top.

Bake at 425°F for 5 minutes to set the exteriors and streusel, then reduce the oven temperature to 350°F and bake for an additional 12-15 minutes until the muffins are golden brown and a toothpick inserted into the center comes out with just a few moist crumbs. Let the muffins cool in the tin for 10 minutes, then transfer to a wire rack to cool completely. The initial high heat helps prevent the streusel from burning while the longer, gentler bake ensures the centers cook through evenly.