2 yellow onions, finely diced (about 3 cups)
1 to 2 tablespoons olive oil
1 to 2 tablespoons fresh thyme leaves, or 1 to 2 teaspoons dried thyme
3 cloves garlic, minced (or ¼ teaspoon garlic powder)
1 teaspoon kosher salt, or more to taste
15 ounces cannellini beans, rinsed and drained
4 cups reduced-sodium vegetable broth
1 to 2 tablespoons balsamic vinegar, or sherry, red wine, soy sauce or Worchestire sauce
Ground black pepper
Chopped parsley for garnish
Liberally mist a large pot with nonstick oil spray and warm over medium-high heat.
Add the mushrooms and leave them undisturbed for 4 minutes. Cook for another 15 to 20 minutes, stirring occasionally.
As they cook, the mushrooms will generate a lot of liquid, and then naturally absorb it as they continue to cook down. The goal is browned and caramelized mushrooms without any remaining liquid. When they’re done, remove a few tablespoons and set aside to use as garnish.
Add the onions, oil, thyme, garlic, and salt and cook for about 5 minutes, until the onions are softened and slightly browned. Add the beans and cook, stirring, for 1 to 2 minutes. Stir in the broth and cook for another 5 minutes, reducing the heat to low when it starts to bubble.
Turn off the heat. Using an immersion blender, puree the soup in the pot, leaving lots of bits and pieces of mushroom for texture throughout.