Heat the butter and oil in a medium-large skillet over medium heat. When the butter is melted, add the onion, 1/4 teaspoon salt, and black pepper. Cook until the onion is deep caramel in color, about 15 to 20 minutes, stirring occasionally. Add a splash of water or turn the heat down a little at any time if the onion or the pan starts to get too dark. Once the onions are caramelized, add the garlic and thyme and cook 1 minute more, stirring constantly.
While the onions caramelize, pour the chickpeas along with their liquid into a medium saucepan. Bring to a simmer over medium heat, and then drain off and discard the liquid.
Pulse the chickpeas in a food processer a few times. Add the tahini, lemon juice, and remaining 1/4 teaspoon salt, and pulse a few times. Add the water one tablespoon at a time with the food processor on, and continue processing until you achieve your desired consistency.
Pulse in 3/4 of the caramelized onion (you can process it until completely smooth or leave it with some texture), reserving 1/4 for topping. Taste and season with additional salt and pepper as desired.
Transfer the spread to a shallow dish and top with the reserved caramelized onion.
Serve warm with a generous sprinkling of cracked black pepper on top, if desired.