Heat half the oil in a small frying pan over high heat. Add the onion, garlic and fennel seeds and cook for 4–5 minutes or until softened. Place the onion mixture, black quinoa, beetroot, quinoa flakes, dill, egg, flour, polenta salt and pepper in a food processor and pulse until just combined. Shape into 6 patties and then dust in remaining polenta.
Place the avocado, feta and lemon juice in a small bowl and mash with a fork until combined.
Heat the remaining oil in a large non-stick frying pan over medium heat. Add the patties and cook for 3–4 minutes each side or until golden brown.
Divide the avocado mixture between the rolls. Top the bases with the alfalfa, patties and pickle. Sandwich with the roll tops and serve with extra pickles.
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