Heat grapeseed oil over medium-high heat in a large skillet. When the oil begins to shimmer, add the onion and garlic and saute until the onion turns translucent. Reduce heat to medium and add the zucchini, yellow squash, oregano, salt, and pepper, and stir to coat. Cover and gently cook for about 15 minutes, then add the corn, cover and cook for about 5 minutes. Meanwhile, make a slurry by adding the flour to a small bowl and gradually whisking in 3 tablespoons of the milk (reserving the rest of the milk for the eggs). Whisk tomato paste into the slurry until smooth. Gently stir the slurry into the squash mixture to thicken. Further cook uncovered until the squash is tender and the flour is "cooked out," about 5 more minutes. Remove pan from heat and let cool to room temperature.
Preheat oven to 350 degrees F. Heat butter over medium heat in another large skillet (or the same one if you have removed the vegetables and washed it). In a small bowl whisk 2 eggs and 1 tablespoon milk together at a time (reserving the rest of the eggs and milk). Spread the melted butter over the bottom of the pan and pour in the eggs, swirling to coat the bottom. Cook as you would a crepe, loosening the edges and flipping to brown both sides. When just cooked, transfer to the bottom of a lasagne dish as the bottom layer and trim and rearrange the egg to fit. Spoon 1/4 of the squash mixture evenly over the eggs, followed by 1/4 of the sliced tomatoes and 1/2 cup of the shredded cheese. Repeat by whisking 2 more eggs with 1 tablespoon of milk and cooking to make a sheet of egg for the next layer, followed by squash, tomato and cheese each time, until you have completed 4 layers.
Heat in the oven for about 20 minutes until the cheese is melted. Remove from oven and let rest 10 minutes before slicing into squares.
Serve sprinkled with a dollop of sour cream and minced fresh parsley.