In a large skillet, heat 1 ½ tablespoons olive oil and butter over medium-high heat. Add the sliced onions and ¼ teaspoon salt and black pepper. Sauté for 12-15 minutes, stirring occasionally, or until onions are soft and caramelized. Stir in mushrooms and cook for 3-4 minutes, or until softened. Stir in garlic and cook for 1-2 minutes. Remove from heat and set aside to cool.

Place half of the cooled onion and mushroom mixture in a food processor or high power blender. Add remaining olive oil, garbanzo beans, tahini, cumin, lemon juice and zest. Blend on high, mixing just until mixture becomes somewhat smooth and combined, but do not over mix. Add water, 1-2 tablespoons at a time as necessary, to ease blending process.

Pour mixture into a bowl and stir to combine. Season to taste with remaining salt and pepper. Top prepared hummus with remaining sautéed onions and mushrooms. Serve immediately with fresh vegetables or multi-grain chips or refrigerate in an air tight container for 3-4 days.