Make the miso broth: Add the dried porcini mushrooms, warm water, miso paste, and tamari to a saucepan. Simmer over low-medium heat while you prep the mushrooms.
Prep the mushrooms: Clean the mushroom caps with a mushroom brush or a damp paper towel. Remove the shiitake stems. Cut the brown, button mushrooms and shiitake into quarters. Slice the oyster mushrooms.
Sauté the onion & garlic: Finely chop the onion and mince the garlic. Heat a large pot or Dutch oven with olive oil. Sauté the onion for 2 minutes over medium-high heat and stir until fragrant. Add in the garlic.
Sauté the mushrooms: Add the mushrooms to the pot. Stir until coated with oil and sauté for 7 minutes over medium-high heat, stirring regularly until they are soft and golden.
Simmer: Pour the miso broth into the pot through a fine-mesh sieve to remove the porcini mushrooms. Stir in the coconut oil and the drained and rinsed lentils. Add in a bay leaf and simmer for 45 minutes over medium heat.
Adjust seasoning with salt if needed. Serve over creamy mashed potatoes.