If you use dried beans and chickpeas, not canned, soak them for about 8 hours. Then strain and boil the chickpeas for 40 minutes and the beans for 45 minutes, or until softened and ready.
Put the ingredients for the flaxseed in a cup or bowl. Stir and set aside for 5 minutes.
Boil the rice for 30 minutes. While it boils, wash and chop the vegetables and herbs.
When the legumes and rice are ready, purée all the ingredients for the burgers in a bowl using a fork (be careful, only 2 Tbsp of olive oil). The final result should have a bit of a rough texture. That is, the mixture should have some larger pieces of beans and whole grains of rice. Avoid using a blender so they don't turn into completely puree and lose the desired rich texture.
Put some olive oil in your palms and shape the burgers tightly. They should have the size of an average burger and be 2 cm/0.7 in thick.
Heat the rest of the olive oil in a large skillet over medium to low heat. Put the burgers in and fry for 10 minutes on the one side (covered) and another 10 minutes on the other side (uncovered) or until they're browned.
Serve in round and fluffy bun. Cut the burger bun in half. On a plate place the bottom bun and top with this order: the avocado sauce, the burger, a tomato slice, an iceberg leaf, 4 onion rings and finally the top bun that you've spread with mustard.
Notes
For unpeeled chickpeas, after you soak them all night, strain and put in a clean towel. Sprinkle a teaspoon of baking soda, close the towel tight and scrub the chickpeas on a table until they are peeled. Open the towel, remove the peels and rinse them in a strainer.
If you want to avoid canned beans, you have to look ahead the day before by soaking and boiling the black beans and chickpeas as well as the rice. That way, the burgers will be ready in 35 minutes.
If you want the mixture to tighten in order to shape the burgers easier, put it in the fridge for 10 minutes. Otherwise, shape the burgers and then refrigerate for 10 minutes to avoid dissolving when frying.