Whiz together all sauce ingredients in a mini-blender and pour into a small crock; cover with plastic wrap and refrigerate.
Bring 3 cups water to the boil in a small saucepan; add brown rice and boil for 30 minutes; add lentils and boil for 20 minutes longer (the mixture will look like porridge); allow to cool (mixture will thicken as it cools).
Put minced onion, grated sweet potato and zucchini into a colander and toss with 1 tsp salt; allow to drain for ½ an hour; after half an hour, take handfuls of the vegetable mixture and squeeze as much water out as you can from it.
Place it into a large bowl as you squeeze it dry; add cooled rice/lentil mixture, minced garlic, cheeses, parsley, dill, mint, egg and rye flour; with hands combine very well so that everything is evenly distributed; refrigerate for at least 30 minutes before frying.
Pour enough oil (I used olive) into a large, non-stick skillet to come ¾ of an inch up sides of pan; heat over medium-high heat until small amount of latke mixture put in will immediately come to the surface and fry fairly briskly.
Take about 2 tbsp of latke mixture at a time, shape into a patty that is about 3 inches in diameter and ½ an inch thick; fry 4-5 minutes; after 4-5 minutes, gently turn patty over and fry other side; as patties turn a deep golden-brown, remove from pan and drain on paper towels.
Repeat with remaining latke mixture until all the patties have been fried.
Serve warm or at room temperature with the sauce.