Preheat the oven to 450°F and spray an 18x13-inch sheet pan with olive oil.
In a large bowl, whisk together the flour, baking powder, baking soda, salt and sugar. Add the butter and use your fingers or a pastry blender to cut it into the flour mixture until it resembles coarse crumbs.
In a medium bowl, whisk together the kefir and eggs. Pour the wet ingredients over the dry ingredients and stir gently with a wooden spoon until just combined. Do not overmix; some lumps are okay.
Spread the batter evenly on the prepared sheet pan, smoothing the top with a spatula. Scatter the banana slices and berries over the batter, pressing them lightly into the surface.
Bake for 10 to 12 minutes or until golden and a toothpick inserted in the center comes out clean.
Cut into squares and serve with sour cream, honey and more fresh berries if desired. Enjoy!