Preheat the oven to 350°F (175°C).

Place the diced bacon in a large skillet over medium heat. Cook for 8–10 minutes, stirring occasionally, until the bacon is crispy and fat has rendered.

Using a slotted spoon, transfer the bacon to a paper towel-lined plate, leaving about 2 tablespoons of bacon fat in the skillet.

Add the diced onion to the skillet and sauté over medium heat for 5–7 minutes, until translucent and lightly caramelized. Remove from heat.

In a small bowl, whisk together brown sugar, ketchup, apple cider vinegar, mustard, salt, black pepper, and garlic powder until smooth.

Drain and rinse the kidney beans, pinto beans, black beans, great northern beans, and chickpeas. Transfer all beans to a 9x13 inch baking dish.

Add the cooked bacon and sautéed onion to the beans and stir gently to combine.

Pour the prepared sauce over the bean mixture and stir until all beans are evenly coated.

Bake uncovered at 350°F (175°C) for 40–45 minutes, or until the casserole is bubbling around the edges and the top is slightly caramelized.

Remove from the oven and let rest for 5 minutes before serving.