The night before hummus time, put ½ cup chickpeas with ½ teaspoon baking soda in a large bowl and cover with cold water at least twice their volume. Cover and soak at room temperature overnight. The chickpeas will double in volume.
The next day, drain and rinse the chickpeas and put them in a large pot with ½ teaspoon baking soda and cook for three minutes, constantly stirring. Add cold water to cover the chickpeas by at least 3 inches and bring to a boil. Skim off foam and cook for about 40 minutes until chickpeas are very tender and but not quite falling apart.
Drain the chickpeas (reserve the cooking liquid, cool and add ice to it to make it very cold) and place them in a food processor and puree. Then add the lemon juice, tahini, and garlic and puree. Then add cumin, olive oil and 3 tablespoons cold cooking liquid and puree until very smooth. Add more liquid if needed to get desired texture. Season with salt and pepper to taste and more lemon juice if needed.
Then make the topping. In a medium sauté pan, heat 1 tablespoon olive oil over medium heat. Add pastrami and sauté for a minute or two until the fat starts to render and the pastrami just starts to crisp. Add leeks and sauté for 2 minutes until they start to soften and then add mushrooms and smoked paprika and sauté until the veggies are tender. Season the mixture with salt and pepper to taste (the pastrami may bring enough salt).
Serve warm or room temperature over the hummus with a drizzle of olive oil and more smoked paprika with pita and sliced veggies. You can make the topping ahead of time and assemble right before serving.