Stir the flour, 2 cups of the sugar, and salt in a large bowl until well combined. Add the shortening or butter and cut it into the flour with a pastry blender or your fingertips until the mixture looks like coarse oatmeal.
Pour half of the mixture into a inch cake pan. Using your fingers or a rubber spatula, press the mixture evenly into the bottom of the pan. Bake for 10 minutes. Remove the pan from the oven and set it aside.
Combine the zucchini and lemon juice in a large pot over high heat and cook until zucchini is tender, 8 to 10 minutes. Stir in the remaining 1 cup of sugar, cinnamon, and nutmeg. Simmer for 1 minute more. Stir in 1/2 cup of the reserved flour mixture and continue to cook, stirring constantly, until mixture thickens. Remove the pot from the heat to cool for 10 minutes.
Pour the zucchini mixture over the baked crust and sprinkle with the remaining flour mixture. Return the pan to the oven and bake until it is lightly browned and bubbly, 40 to 45 minutes.