Prep: Dice onion, carrots, celery; mince garlic; roughly chop mushrooms. Rinse lentils until water runs clear. Measure out broth and seasonings.
Heat oil: Warm a large heavy-bottomed pot or Dutch oven over medium-high heat. Add 2 tbsp olive oil.
Sauté aromatics: Add diced onion with a pinch of salt and cook 4–5 minutes until translucent and beginning to colour. Add carrots and celery and cook 4 minutes more. Stir in minced garlic and cook 30–45 seconds until fragrant.
Brown mushrooms: Add chopped mushrooms. Sear without overcrowding, 6–8 minutes, until they release liquid and then brown. If needed, do this in batches to ensure colour.
Build flavours: Stir in 2 tbsp tomato paste and cook 1–2 minutes to caramelize. Add 1 tbsp soy sauce/tamari and scrape up any browned bits from the bottom of the pot. Add the diced tomatoes and 2 cups of the vegetable broth, stirring to combine.
Add lentils & simmer: Add rinsed lentils, thyme, smoked paprika (if using), and the bay leaf. Add the remaining 2 cups broth (total 4 cups). Bring to a boil, then reduce heat and simmer uncovered for 25–30 minutes, stirring occasionally, until lentils are tender but not falling apart. If stew becomes too thick, add up to 1 cup more hot broth or water.
Finish & balance: Off the heat, stir in 1 tbsp balsamic vinegar. If using, dissolve miso in a little hot broth and stir in now, or stir in 1/4 cup coconut milk/oat cream for creaminess. Taste and adjust salt and pepper. Remove and discard bay leaf.
Rest & serve: Let the stew rest 5 minutes to meld flavour. Garnish with chopped fresh parsley and serve over mashed potatoes, polenta, grains, buttered noodles, or with crusty bread.