Preheat the oven to 350 degrees F.
In a large bowl, beat together 1 egg with a generous pinch of salt and pepper. Whisk in 1 tablespoon canola oil. Add half the spaghetti (reserve the other half for Round 2 Recipe Spinach and Mushroom Pasta). Toss it well to coat and press it into a 9-inch deep dish pie pan to form a crust.
In a skillet over medium-high heat, add the remaining oil. When it is hot, add the onion and cook until it is softened, about 5 minutes. Add the garlic and cook for 1 minute. Remove from the heat and set aside (reserve half the cooked onions for the Round 2 Recipe Spinach and Mushroom Pasta).
In a large bowl, whisk together the remaining 2 eggs, milk, Italian seasoning, and salt and pepper, to taste. Stir in the cheese, spinach (reserve 3/4 cup spinach for Round 2 Recipe Spinach and Mushroom Pasta), and remaining half of the cooked onions. Pour the mixture on top of the spaghetti pie crust, top with tomato slices, and sprinkle with Parmesan. Place onto a baking sheet and bake until the pie is set and golden brown, 30 to 35 minutes. Remove and let rest for 10 minutes before slicing.