FOR CITRUS RELISH: In a bowl, gently stir together the orange and grapefruit segments with the orange and lime zest, along with the sugar. If desired, add a little orange juice to thin for more liquid.
1. Heat the oven to 350 degrees. Place the almonds in a food processor and pulse to a medium or fine grind.
2. Prepare the muffin tins: Spray each muffin tin with vegetable spray and dust with a coating of sugar. Sprinkle about 2 teaspoons of ground almonds on the bottom of each tin. Set aside.
3. Prepare the flax egg by mixing the flaxseed meal with water and let sit for a few minutes - it should soon replicate the consistency of whisked eggs. In a medium bowl, whisk together the remaining ground almonds along with the flour, baking powder and salt.
4. In the bowl of a stand mixer fitted with the whisk attachment, or in a large bowl using a hand mixer, beat together the orange and lemon zest with the sugar and flex “eggs” until light, fluffy and doubled in volume, 3 to 5 minutes. Slowly beat in the milk, apple cider vinegar and olive oil until incorporated.
5. Gently fold in the flour mixture just until combined. Divide the mixture evenly among the muffin tins, filling each 2/3 to 3/4 full.
6. Bake in the center of the oven until the top is set and the crown is very lightly browned, 15 to 20 minutes. Remove and cool the racks on a tray.
7. While the cakes are still warm, pop them out of the trays and serve, garnished with the citrus relish and yogurt.