1. In a food processor, combine beans, 1/3 cup oil, the tahini, juice, garlic, 1/2 teaspoon salt and 1/4 teaspoon pepper; puree until smooth. Transfer to a platter.
2. In a large nonstick skillet, heat remaining 2 tablespoons oil and the butter over medium-high heat until melted; add leeks. Saute, stirring occasionally, until just golden, about 6 minutes. Add mushrooms and remaining 1/2 teaspoon salt and 1/4 teaspoon pepper; cook until mushrooms are softened, about 5 minutes. Stir in thyme.
3. Top hummus with mushroom-leek mixture and garnish with thyme, if desired. Serve at room temperature. Makes 4-1/4 cups.