Preheat the oven to 425°F and grease a 10-inch baking dish with butter. Set aside. Peel the zucchini and slice it in half lengthwise. Use a spoon to scrape out the seeds.
Cut the seeded zucchini into small pieces and place in a medium-sized saucepan. Add lemon juice, sugar, cinnamon, and nutmeg to the pan and bring to a boil over medium-high heat.
Reduce the heat to maintain a simmer and continue cooking for 15 minutes, stirring frequently. Remove from the heat and pour the zucchini mixture into the prepared baking dish.
Topping Instructions
While the zucchini filling is on the stove, combine the flour, sugar baking powder, salt, and cinnamon for the topping in a bowl and whisk to combine.
Grate the butter into the flour mixture and stir with a fork to mix throughout. Stir in the boiling water, just until combined, leaving plenty of little lumps of butter.
Drop spoonfuls of the topping over the zucchini in the baking dish. Bake until the crust is golden and a toothpick inserted into the crust comes out clean, about 24-28 minutes. Serve warm or at room temperature.