Heat the olive oil or 2 Tbs broth in a 4 to 5 quart Dutch oven or soup pot over medium heat. Add the onions and celery and cook until translucent. Add the potatoes, turnips, carrots, mushrooms, and garlic, stir well and cover pot with a lid. Cook for 10 minutes, or until the vegetables have given off some liquid.
Scootch the vegetables to one side and tip the pot to pool liquids to the cleared area (or, if dry, add a splash of vegetable broth). Spoon the tomato paste into the liquid, and top with the smoked paprika. Stir to create a paste, and let cook until fragrant.
Add a splash of vegetable broth to the pot, and sprinkle the flour over the vegetables. Mix the tomato paste into the vegetables, and stir until the flour has disappeared into the vegetables. Add the beer, and let simmer, to cook off the alcohol.
Add the remaining broth, miso, herbs, and lentils. When the stew begins to boil, reduce heat to maintain a gentle but active simmer (medium-low should do it). Cover partially with the lid, and cook for 45 minutes, stirring occasionally.
Remove bay leaf, and let stew rest on warm/low for 10 minutes before serving.
Stew can be made ahead (even the day before). When rewarming, if the stew is too thick, add vegetable broth by the tablespoon until it reaches the consistency you prefer. Taste, and add salt for seasoning, if desired.
For a winter comforting treat, serve with mashed potatoes.