Dice the onion into medium pieces and chop the celery stalks into 1/2-inch pieces. Peel and cut the russet potatoes into large 1.5-inch chunks, then peel and cut the turnips into similar-sized pieces. Peel and cut the carrots into 1-inch rounds or chunks. Slice the cremini mushrooms into 1/4-inch pieces—this thickness is key for creating that meaty texture that makes vegetarian stew satisfying. Measure out the lentils, rinse them if desired, and set aside. This prep work ensures everything cooks evenly and you're ready to build flavor efficiently.

Heat 1.5 tbsp olive oil in a large pot or Dutch oven over medium-high heat. Add the diced onion and celery, cooking for 5-7 minutes until softened and the onion becomes translucent. This step develops a sweet, savory foundation—don't skip the browning time as it builds essential flavor depth for the stew. Transfer the softened vegetables to your slow cooker.

In the same pot (don't wash it—those browned bits add flavor), add the sliced mushrooms over medium-high heat. Cook for 10-12 minutes, stirring occasionally, until they release their moisture and begin to brown and caramelize. This is where the mushrooms develop that meaty, umami-rich character that gives vegetarian stew its satisfying depth. Once they're browned and any excess liquid has evaporated, sprinkle the flour over them and stir constantly for 1-2 minutes to cook out the raw flour taste and create a light roux.

Pour the Guinness into the pot with the mushroom mixture, using a wooden spoon to scrape up all the flavorful browned bits stuck to the bottom—this technique is called deglazing and it captures concentrated flavor. Let it bubble gently for about 1 minute, then transfer the entire mixture (mushrooms, liquid, and all) to the slow cooker with the cooked onions and celery. I like using Guinness because its slight bitterness and roasted notes add complexity that elevates the entire stew.

Add the prepared potatoes, turnips, carrots, and green lentils to the slow cooker. In a small bowl, whisk together the tomato paste, Dijon mustard, paprika, sugar, salt, and black pepper until smooth, then add this mixture to the slow cooker. Pour in the vegetable broth and add the bay leaves. Stir everything together thoroughly to combine all ingredients and distribute the spices evenly. This assembly ensures every spoonful will have balanced flavor and proper seasoning from the start.

Cover the slow cooker and cook on low for 8-9 hours or on high for 5-6 hours. The long cooking time allows the vegetables to become tender, the lentils to fully cook and thicken the broth naturally, and all the flavors to meld together beautifully. The stew is done when the potatoes and root vegetables are easily pierced with a fork and the lentils are soft.

Remove and discard the bay leaves from the stew. Taste the stew and adjust seasonings if needed—it should be rich, savory, and deeply flavored. Ladle into bowls and garnish generously with fresh chopped parsley, which adds brightness and a fresh herbal note that cuts through the richness of the stew.