In a large bowl combined flour, sugar, baking powder, and salt. Whisk until combined.
Add the diced rhubarb and toss to coat with flour mixture.
Add oil and milk and stir until the dry ingredients are moistened. Be careful not to over mix.
Spread into a greased 8-inch square baking pan. Set aside.
In a medium bowl combine flour, brown sugar, and nutmeg for the crumble topping. Whisk to mix well.
Cut butter into the brown sugar mixture until the mixture is combined and crumbly.
Sprinkle crumble topping onto the coffee cake batter and bake at 375 degrees for 45 to 50 minutes. Test for doneness by pressing a toothpick into the center of the coffee cake if it comes out clean, it's done.