Preheat the oven to 350ºF. Slice the top and bottom knobs off of each zucchini squash, then slice them in half lengthwise. Use a spoon to scoop out the center seeds of each squash, creating a crevice that will hold the black bean filling. Arrange the squash halves, cut-side-up, on a large baking sheet and bake them at 350ºF until very tender, about 20 minutes.
While the squash is baking, heat the olive oil in a large skillet over medium-high heat and saute the onion and pepper until they start to soften, about 5 minutes. Add in the garlic and saute another minute.
Add in the black beans, cumin, salt, and chili powder, and stir until everything is heated through. Turn off the heat and let the vegetables sit in the warm pan until the zucchini is done cooking.
To prepare the "cheese" sauce, stir together all of the ingredients in a bowl until smooth and creamy. Adjust any seasoning to taste. (Keep in mind that it will be slightly salty and tangy, but will be diluted in flavor when drizzled over all those vegetables.)
To assemble, remove the zucchini from the oven and fill each half with plenty of the hot black bean mixture. Top with a generous drizzle of tahini sauce and garnish with fresh cilantro. Serve warm.
Note: If you'd prefer to top with real cheese, return the stuffed zucchini topped with cheese to the oven and broil until the cheese is bubbly, about 1-2 minutes. Cool, then serve.