Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13-inch baking dish. Whisk together 1 1/2 cup of sifted flour, baking powder, and salt in a large bowl.
With a pastry cutter, chop the unsalted butter into the flour mixture until it resembles coarse crumbs; mix in 1 egg and 1 tablespoon of milk, creating a dough; if dough seems dry and crumbly, mix in additional milk, a teaspoon at a time. Pat the dough into the prepared baking dish, and sprinkle the cherries evenly over the dough.
In a bowl, sift 3/4 cup of sugar with 2 tablespoons of flour; whisk in the 3 eggs and 3/4 cup of milk until smooth. Mix in cinnamon and nutmeg if desired; pour the mixture over the cherries. In a small bowl, using a pastry cutter, chop 2 tablespoons of butter with 1/2 cup of sugar and 1/2 cup flour until crumbly; sprinkle the streusel over the custard mixture.
Bake in the preheated oven until the custard is set and the streusel is lightly browned, 25 to 30 minutes. Serve warm or cold; refrigerate leftovers.