Combine the yogurt, sour cream, cucumber, and garlic in the food processor and puree until smooth. Add the mint olive oil, lemon zest and juice. Give it a couple of more pulses and transfer to a bowl. Season with salt and pepper to taste. Keep refrigerated until ready to use.
PREPARING THE ZUCCHINI PANCAKES
Place the coarsely grated zucchini in a colander. Sprinkle with salt and let stand for 30 minutes. Squeeze the zucchini to remove as much liquid as possible. Dry the zucchini by pressing it between several layers of paper towels.
In a large bowl, combine the zucchini, mixed scallions, eggs, flour, parsley, mint, tahini, salt, and pepper until well blended. Fold in the feta cheese and the walnut to the zucchini mixture.
Preheat the oven to 300° F to keep the pancakes warms while you are cooking them.
Add enough oil to a large skillet to a depth of about 1/8 inch, and heat over medium high heat. The oil must shimmer but not be smoking. Working in batches, drop heaping tablespoons of the zucchini mixture into the hot oil, do not overcrowd the skillet. Fry until the pancakes are golden brown and cooked through, about 2 to 3 minutes per side. Transfer each batch of pancakes to the baking sheet in the oven to keep warm. Serve immediately with the tzatziki sauce