The leftover onion marmalade used to flavor this recipe can be spread on sliced bread or served with cheese.
1. To make Onion Marmalade: Heat oil in Dutch oven over medium-high heat. Add onions, and cook 30 minutes, stirring occasionally. Stir in vinegar, brown sugar, and thyme, and reduce heat to medium-low. Cook 40 minutes, or until onions are caramelized. Cool.
2. To make Cake: Preheat oven to 350°F. Line loaf pan with parchment paper, letting ends hang over pan edges. Spray parchment with cooking spray.
3. Whisk together flour, baking powder, salt, and Aleppo pepper in bowl. Whisk together eggs, yogurt, and oil in separate bowl. Stir egg mixture into flour mixture. Fold in spinach, 1/2 cup Onion Marmalade, and walnuts. Spoon batter into prepared loaf pan, and smooth top with spatula. Bake 45 minutes, or until tip of knife inserted into loaf comes out clean. Lift loaf from pan with parchment paper, and cool on wire rack. Serve with remaining Onion Marmalade, if desired.