Combine the black beans, water, minced mushrooms, bay leaves, salt, and baking soda in a large pot and bring to a boil over high heat. Reduce the heat to low and simmer until the beans are tender, about 90 minutes to 2 hours. Taste several beans to ensure they are creamy throughout; if they are still firm, continue simmering until fully softened.

Heat the olive oil in a Dutch oven over medium-high heat until the oil shimmers but does not smoke. Add the onions, celery, and carrot and cook until the vegetables are softened and just beginning to turn golden brown, about 10 to 12 minutes.

Stir in the garlic, cumin, and red pepper flakes and cook until fragrant—the heat will wake up the spices without letting them burn.

Add the cooked beans with their cooking liquid and the vegetable broth to the Dutch oven. Bring the mixture to a simmer and cook for 30 minutes to allow the beans to release their starch and the broth to darken and thicken.

Whisk the cornstarch and water together in a small bowl until smooth. Stir the slurry into the simmering soup and cook until the liquid thickens enough to coat the back of a spoon and looks glossy, about 2 minutes.

Remove the bay leaves and stir in the lime juice. Season with salt and pepper to taste and serve hot, ideally topped with cilantro, diced red onion, avocado, and sour cream.