In a large bowl, mix mashed bananas and melted butter, using wooden fork or spoon. Mix in the sugar, egg, and vanilla. Sprinkle the baking soda and salt over the mixture and mix in. Add the flour last, mix just enough to blend the ingredients; do not over-mix, the lumps will work themselves out.
Add lemon zest, chopped strawberries and whole blueberries to the batter and mix them in to distribute evenly through the batter.
Butter a 4×8 inch loaf pan. Line the bottom of the pan with parchment paper, butter the parchment paper too. Pour the batter in the pan. Bake for 1 hour. Cool on a rack. Remove from pan and slice to serve.
Note: lining the bottom of the pan with parchment paper ensures that the bottom of the bread will not get stuck.