In a large bowl, add the sugar and zest the oranges over it. Rub the zest and sugar between your fingers until it’s well-distributed and becomes like wet sand.
Add the eggs and using a hand mixer, beat the mixture until very light and fluffy, about 3-5 minutes. With the mixer on, slowly drizzle in the olive oil. Beat to combine.
Add the flour, vanilla, salt and baking soda and fold gently. Then, with the mixer on drizzle in the kefir and orange juice, beating until smooth. Cover the bowl with plastic wrap and set at room temperature for 1-2 hours or in the fridge overnight.
Once it’s time to bake, preheat the oven to 400 F. Line two 12 cup muffin trays with liners, six in each, alternating cavities so that no muffins are next to each other. Portion the batter into the cups, filling ¾ of the way. Sprinkle granulated sugar on top and bake for about 18 minutes, or until you can press the top of a muffin and it springs back.