Line the inside of a 5-quart slow cooker with foil, and lightly coat with nonstick cooking spray. If any lands on the rim where the lid rests, wipe clean with a paper towel.
To prepare the crumble, whisk together the oats, flour, and cinnamon in a medium bowl. Add in the maple syrup and butter, mixing until completely incorporated and no dry bits remain.
To prepare the cake, whisk together the flour and next 6 ingredients (through salt) in a medium bowl. In a separate bowl, whisk together the butter, egg white, and vanilla. Mix in the pumpkin purée. Add in the lemon juice and maple syrup, stirring until fully incorporated. Gradually mix in the flour mixture, stirring just until incorporated.
Spread the cake batter into the prepared slow cooker, and sprinkle the crumb topping over the top. Gently press the crumb topping into the cake batter. Cover and bake on “Low” for 2 hours 15 minutes. Turn off the slow cooker, and let the coffee cake cool in the pot for 10 minutes before pulling it out by gripping the edges of the foil. Place on a wire rack, slide the foil out from underneath, and let the coffee cake cool completely to room temperature before serving.
Notes: For a vegan version, substitute coconut oil or Earth Balance Buttery Spread in place of the butter, and add an additional 3 tablespoons of pumpkin purée to the cake in place of the egg white.
Regular whole wheat flour may be substituted in place of the whole wheat pastry flour. The cake may be a tad bit denser. (All-purpose flour may be substituted in place of both whole wheat flours as well.)
The coffee cake batter isn’t too sweet because it’s meant as a breakfast cake. If you prefer a sweeter taste, increase the maple syrup to ½ cup instead.