Bring 125 ml of water to a boil. Slice the onion into thin rings using a mandolin if you have one and put them in a bowl. Pour the vinegar and the hot water into the bowl, add the caster sugar and leave to marinate for at least 30 minutes.
Preheat the oven to 200 °C and if you want to cook them on the BBQ, light the BBQ. Grind the oatmeal flakes finely into flour in the food processor and scoop out of the mixing bowl.
Peel and remove the base of the beets. Grate them coarsely in a food processor or with a box grater. Rinse the black beans and add them to the grated beetroot in the food processor and purée until everything is coarsely chopped and becomes a coarse mixture.
Spoon the beetroot mixture into a mixing bowl and press the garlic onto it. Add two tablespoons of olive oil, the paprika and the cumin. Add the oatmeal flour a little at a time and mix until it takes on a dough texture. Season with salt and pepper.
Divide the beetroot mixture into six equal portions. Shape them into burgers with moist hands and place them on a baking tray lined with baking paper. Brush them with the rest of the olive oil.
Bake the burgers in the oven for about 20 minutes. If you want to finish them on the BBQ, deduct 5 minutes from the oven time and put them on an aluminium BBQ tray on the BBQ grill for about 8 minutes. Turn the burgers halfway .
Meanwhile, make the avocado mayonnaise. Zest the peel of the lime. Squeeze the juice. Cut the avocado all the way around the stone. Twist the two halves apart. Remove the stone, scoop out the flesh with a spoon and put it in the blender or in the bowl of the hand blender. Add the lime zest and the mayonnaise and purée until smooth. Season with lime juice and salt and pepper.
Slice the gherkins lengthwise. Cut the hamburger rolls in half and grill them on the BBQ with the cut side down for 30 seconds. Brush one half of the roll with avocado mayonnaise, divide the spinach over the rolls and place the burgers on top. Garnish with the gherkin, spoon more avocado mayonnaise on top and add a sprinkle of pickled onion. Cover with the other half of the roll.
Tip: Make a large batch of the pickled onions. Put them in a perfectly clean, boiled-out jam jar, add some herbs such as coarsely chopped dill or a few sprigs of thyme. This can be kept in the refrigerator for at least a week.
