While you're gathering ingredients, heat the vegetable oil in a large skillet over medium-high heat. Pat the chicken thighs dry with paper towels—this helps them brown better and develop a flavorful crust. Once the oil is shimmering, working in batches if needed, sear the chicken thighs skin-side down for about 5 minutes until golden brown. You're not cooking them through, just building flavor. Transfer the browned chicken to your slow cooker. Meanwhile, finely chop the onions, mince the garlic, slice the mushrooms, peel and slice the carrots, dice the celery stalks, and chop the fresh parsley. Having everything prepped will make the next steps flow smoothly.

In the same skillet used for the chicken (don't clean it—those browned bits add flavor), melt the butter over medium heat. Add the chopped onions and cook for about 5 minutes, stirring occasionally, until they become soft and translucent. Add the minced garlic and cook for 1 minute until fragrant. Now sprinkle the flour over the mixture and stir constantly for 2 minutes—this cooks out the raw flour taste and creates a roux that will thicken your sauce. Add the salt, black pepper, thyme, and celery salt, stirring to combine. I like to let the spices toast slightly with the flour to deepen their flavors.

Pour the chicken stock and lemon juice into the skillet with the aromatic mixture, stirring well to combine. Bring the mixture to a gentle boil for about 1-2 minutes, scraping up any browned bits from the bottom of the pan. Pour this flavorful sauce over the browned chicken in the slow cooker. Add the sliced mushrooms, sliced carrots, and diced celery to the slow cooker and stir everything together, ensuring the vegetables are submerged in the liquid as much as possible. Add the bay leaf for additional depth of flavor.

Cover the slow cooker and cook on low for 5-6 hours, or on high for 3-4 hours, until the chicken is very tender and pulls apart easily with a fork. I prefer the low setting because it develops richer flavors and keeps the vegetables from falling apart. The longer, gentler cooking breaks down the collagen in the thighs, making them incredibly tender and flavorful.

Once the chicken is cooked through and tender, remove it from the slow cooker using tongs or a slotted spoon and set it aside on a cutting board. Using two forks or your hands, shred the chicken into bite-sized pieces, discarding the skin and bones. Stir the cream into the sauce in the slow cooker, then return the shredded chicken to the slow cooker and gently fold it in to combine. Taste and adjust seasoning if needed. Divide the casserole among serving bowls and garnish generously with the freshly chopped parsley.