Move an oven rack to the middle position and preheat oven to 350 degrees. Coat a 9-inch by 13-inch cake pan with nonstick spray.
In a large bowl, whisk together flour, brown sugar, granulated sugar, nutmeg, and salt. Stir in vegetable oil until combined (the mixture will be crumbly). Measure out 1/2 cup of the batter and reserve for later to use as topping.
In a medium bowl, whisk together buttermilk, egg, cinnamon, baking powder, and baking soda until smooth. Pour into the bowl with the dry ingredients and stir until just combined. Do not over-mix.
Pour into prepared cake pan. Sprinkle with reserved crumb mixture. Bake until the top is set and a toothpick inserted in the middle comes out clean with a few crumbs attached, about 40 minutes. Remove from oven and cool in pan for 5 to 10 minutes.
Notes
Buttermilk: To make your own buttermilk, stir together 1 c. milk + 1 Tbsp. lemon juice or white vinegar. Set aside until thickened and curdled, about 10 minutes.
Yield: This recipe makes a 9-inch by 13-inch coffee cake, enough for 12 generous slices (more or less depending on how you slice it).
Storage: Store leftover coffee cake at room temperature for up to 5 days. Store in an airtight container or wrap individual slices tightly in plastic wrap.