Caramelize onions in olive oil over medium-low heat until nicely browned, about 30 minutes.
Preheat oven to 350 degrees. Line a 9 x 5-inch loaf pan with parchment paper, hanging over on two sides. Spray pan well with cooking spray.
In a medium bowl whisk together sour cream, vegetable oil and eggs. In a large bowl, whisk together remaining dry ingredients. Fold in caramelized onions and stir until well incorporated. Add wet ingredients to dry and stir to combine.
Pour batter into prepared pan. Bake for 40 - 50 minutes until a toothpick in center comes out clean (ours went the 50 minutes).
Let cool 1 hour then remove loaf from pan and transfer to wire rack to cool completely.