Preheat oven to 450 degrees. Prepare two rimmed baking sheets with parchment paper for easy clean-up. Oil a 2-quart baking dish with olive oil.
In a medium saucepan over medium-high heat, add 1 tablespoon olive oil and heat until shimmering. Add minced garlic and stir until fragrant, about 30 seconds. Add crushed tomatoes (Be careful! They may splatter when they hit the hot oil, so keep the lid handy), basil, oregano, and red pepper flakes (if using). Bring the sauce to boil over medium-high heat.
Reduce heat to medium-low, cover, and simmer until the flavors have blended, about 10 to 15 minutes longer. Season to taste with salt and pepper and keep warm over medium-low heat.
Meanwhile, in a large bowl, add zucchini slices. Add 2 tablespoons olive oil, season with salt and freshly ground black pepper (I like 1/2 teaspoon salt and 1/4 teaspoon pepper). Toss to combine. Arrange zucchini slices on prepared baking sheets in an even layer. Bake until zucchini is light golden brown and fork-tender, about 12 to 15 minutes.
Reduce heat to 375 degrees. Spread 1/4 cup tomato sauce over the bottom of the prepared casserole dish. Arrange one-third of the zucchini in an even layer over the tomato sauce. Spoon one-third of the remaining tomato sauce over zucchini. Sprinkle 1/4 cup Parmesan cheese over layer, followed by 1/4 cup mozzarella. Repeat layers with remaining 2/3 zucchini slices, tomato sauce, and cheeses.
Bake until bubbling and cheese is browned and bubbling, about 30 to 35 minutes. Remove from oven and allow to rest 5 minutes before serving. Garnish with fresh parsley, if desired.