Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add potatoes, cauliflower, and onion; cover and steam until tender, 10 to 15 minutes.
Blend steamed vegetables, flour, Cheddar cheese, milk, eggs, mustard, garlic, salt, and pepper in a blender until batter is thick; transfer to a bowl. Stir red pepper into batter.
Spray a hinged griddle or flat griddle with cooking spray and turn heat to medium. Spoon batter, 1 to 2 tablespoons per cake, onto the hot griddle; close griddle or flip cakes and cook until lightly browned, about 4 minutes. Repeat with remaining batter.