Preheat the oven to 425 F (220 C) and place the rack in the middle.
Wipe the portobellos with a damp paper towel if they need to be cleaned. Gently remove the mushroom stems and use a spoon to scoop out the gills on the underside of the mushroom.
In a small bowl, mash half (¾ cup) of the chickpeas with a fork or a potato masher.
Mix in the lemon juice and tahini and stir until it smooths out.
Add the garlic, sumac, thyme, salt, and 1 teaspoon of sesame seeds.
Stuff each mushroom with the chickpea mixture. Use the back of a spoon to create a small cavern in the filling so the whole chickpeas don't fall off.
Top each mushroom with the remaining chickpeas, and sprinkle with the rest of the sesame seeds.
Place the mushrooms in a shallow baking dish or on a baking tray.
Bake for 20 – 30 minutes or until the mushrooms are tender when pierced with a fork.