Step 1: Sear the Chicken (The Flavor Key)
Pat the chicken thighs completely dry with paper towels. Season generously on both sides with salt and pepper. Heat the olive oil in a large skillet over medium-high heat. Once hot, add the chicken (in batches if needed) and sear for 2-3 minutes per side, until a deep golden-brown crust forms. This step is crucial for flavor and is not just for looks. Transfer the seared chicken to the bowl of your slow cooker.

Step 2: Sauté Aromatics (Optional but Recommended)
In the same skillet (don’t wash it), add the diced onion. Cook over medium heat for 3-4 minutes until starting to soften. Add the minced garlic and cook for 1 more minute until fragrant. Pour in the chicken broth to deglaze the pan, scraping up all the flavorful browned bits (fond) from the bottom. Let it simmer for 1 minute, then remove from heat.

Step 3: Build the Base in the Slow Cooker
Pour the onion-garlic-broth mixture over the chicken in the slow cooker. Add the juice of the two lemons, thyme, oregano, onion powder, and red pepper flakes (if using). Stir gently to combine. Place the thin lemon slices on top of the chicken. Cover and cook on LOW for 5-6 hours or on HIGH for 3-4 hours. The chicken is done when it is tender and easily shreds with a fork.

Step 4: Thicken the Sauce
About 30 minutes before serving, prepare a slurry. In a small bowl, whisk the flour (or cornstarch) with the ¼ cup of cold water until completely smooth. Remove the chicken from the slow cooker to a plate and shred it into bite-sized pieces using two forks. Whisk the slurry into the hot liquid remaining in the crock. Cover and turn the slow cooker to HIGH. Let it cook for 20-25 minutes, until the sauce has thickened noticeably. This step creates the perfect base for the creamy finish.

Step 5: Create the Creamy Lemon Finish
Reduce the slow cooker to the WARM setting (or turn it off). Stir in the room-temperature heavy cream, grated Parmesan, and lemon zest until fully combined. The residual heat will warm the cream without curdling it. Add the shredded chicken back to the sauce and stir to coat. For a final touch of richness, add the cold butter, stirring until it melts and makes the sauce extra glossy.

Step 6: Final Seasoning and Rest
Taste the sauce critically. Adjust seasoning with additional salt and a pinch of white pepper (which is more delicate than black). Let the dish sit, covered, for 5-10 minutes to allow the flavors to fully marry and the sauce to reach its perfect, creamy consistency.

Step 7: Garnish and Serve
Serve the creamy lemon chicken over your chosen base—linguine, rice, mashed potatoes, or roasted asparagus are all excellent choices. Garnish generously with fresh chopped parsley and an extra sprinkle of lemon zest. Add a fresh lemon slice on the side for those who love an extra citrus punch.