Preheat the oven to 350°F. Line a 9 x 13-inch baking dish with parchment paper.
Heat the olive oil in a large skillet over medium heat. Add the chopped kale, garlic, and ½ teaspoon salt. Cook for 5 minutes, stirring often, or until completely wilted and tender. Let cool.
In a large bowl, whisk together the eggs and Greek yogurt until smooth. Stir in the cooked kale, ¾ cup parmesan, black pepper, and lemon zest, if using.
Add the oats to a high-speed blender or food processor and blend briefly into a coarse meal. Add the walnuts and pulse into a meal, being careful not to blend them into nut butter.
Add the oat-walnut mixture, baking soda, and remaining ¼ teaspoon salt to the egg mixture. Stir until combined.
Pour into the prepared baking dish and spread it into an even layer. Sprinkle the remaining ¼ cup Parmesan evenly over the top.
Bake for 25-30 minutes, or until the center is set, the tops are lightly golden, and a knife inserted into the middle comes out clean.
Let cool for at least 10 minutes before lifting from the pan and slicing into 12 squares.