Prepare the oven and pan: Preheat your oven to 350°F (175°C) and grease a 9x5-inch loaf pan. Line it with parchment paper, leaving a little overhang for easy removal.

Make the strawberry swirl: In a small saucepan, combine the ¾ cup chopped strawberries, 2 teaspoon sugar, and ½ teaspoon cornstarch. Cook over medium heat for about 3 minutes, stirring constantly, until thickened and jam-like. Remove from heat and let cool completely.

Mix dry ingredients: In a medium bowl, whisk together 1½ cups of the flour, baking soda, salt, and cinnamon. Set aside. Reserve the remaining ½ cup flour for coating the strawberries.

Coat the strawberries: Toss the 1 cup diced strawberries with the reserved ½ cup flour until evenly coated. This helps prevent them from sinking in the batter.

Cream butter and sugar: In a large bowl, beat the softened butter and brown sugar until light and fluffy. Add the eggs one at a time, mixing well after each addition, then stir in the sour cream and vanilla extract until smooth.

Combine batter: Stir in the mashed banana. Gently fold in the flour-coated strawberries, mixing until just combined.

Layer and swirl: Spread half of the batter into the prepared loaf pan. Spoon the cooled strawberry jam on top and swirl it gently with a knife. Add the remaining batter on top and smooth the surface.

Bake: Bake for 55–65 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool in the pan for 10 minutes, then lift out using the parchment overhang and cool completely on a wire rack.