Preheat the oven to 375°F and set an oven rack in the middle and upper third of your oven. Spray a 13×9 baking dish with cooking spray.
In a large skillet, heat oil over medium heat. Add garlic and cook until fragrant, 1 minute. Add spinach and cook until wilted, 4 minutes. Remove from heat.
In a medium bowl, whisk the eggs. Then stir in the ricotta, ½ cup grated Parmesan, basil, cooked spinach mixture, lemon zest, lemon juice, 1 teaspoon kosher salt, and black pepper.
Arrange ½ of the heirloom tomato slices in a single layer on the bottom of the prepared baking dish, reserving the “prettier” slices for the top of the baking dish.
In the baking dish with the arranged heirloom tomatoes, add a layer of the lasagna sheets (4 per layer), then ¼ of the tomato sauce, then ¼ of the ricotta mixture, and ¼ of the mozzarella cheese. Repeat this sequence 3 more times, reserving the final ¼ of mozzarella cheese. Top final layer of ricotta with remaining ½ of pretty heirloom slices and remaining ¼ of mozzarella and remaining ½ cup grated Parmesan.
Spray a sheet of aluminum foil with cooking spray and lay it, oiled side down, over the lasagna, covering the entire baking dish. Place the lasagna over a sheet pan (to catch any bubbling sauce) and bake in the center rack of the oven until sheets are fully cooked, 35-40 minutes.
Remove the foil and transfer to the top of the oven. Broil until the cheese is golden and crispy, 2-3 minutes.
Garnish with fresh basil and more Parmesan, if desired. Let sit for 10 minutes before cutting and serving.