Preheat Oven to 350 degrees. Cream together the brown sugar and butter for about 3 minutes in a stand mixer or with hand mixer. Add in the eggs one at a time, beating them into the mixture. Add the kefir, bananas and vanilla, beat for 1 minute.
In a separate bowl, mix together the flour, baking soda and salt. Using a large spoon or spatula, slowly mix the dry ingredients into the wet ingredients, leaving about 1 Tbls of the dry ingredients in the bowl. Mix until no flour pockets remain but do not over mix. Add in the strawberries that have been mixed in the remaining flour mixture. Stir mixture until just combined.
Spoon batter into 8x5 loaf pan that has been sprayed with cooking spray. In a separate bowl, add the remaining ingredients for the streusel topping, cut in the butter until it forms small pea sized chunks. Sprinkle over the bread evenly.
Bake for 60-65 minutes, loosely covering the bread at the 30 minute mark with foil. This will make sure the bread doesn't get too dark. Bread is done when toothpick inserted comes out clean.
Take bread out of oven and let cool for 20 minutes on wired rack. Remove bread from pan and let cool.