Prepare and Season the Chicken
In a small bowl, mix together the chicken seasoning: chili powder, garlic powder, lemon pepper, salt, and red pepper flakes. Drizzle the chicken with a little olive oil and season liberally on both sides, ensuring the spice mixture adheres well to create a flavorful crust when browned.
Brown the Chicken
On the sauté function of the slow cooker (or in a skillet over medium heat), add 2 tablespoons olive oil. Once hot, add the seasoned chicken and cook for about 2-3 minutes per side, just enough to achieve a golden-brown crust. This step seals in the juices and enhances the flavor. Remove chicken and set aside.
Prepare the Gravy Base
Still on the sauté mode, add the butter and allow it to melt completely. Add the thinly sliced onions, diced celery, and minced garlic to the melted butter and cook until softened, about 3-4 minutes. The vegetables will release their flavors and begin to become translucent.
Create the Roux
Sprinkle the flour and bouillon powder over the softened vegetables and continue to cook for 1 minute, stirring continuously. This creates a roux that will thicken your gravy and ensure it's lump-free while developing a deeper flavor.
Complete the Dish
Gradually whisk in the chicken stock to create a smooth gravy. Nestle the browned chicken breasts into the liquid, ensuring they're partially submerged. Set the slow cooker function on high for 4 hours or low for 6 hours, allowing the chicken to cook until it easily falls apart when touched with a fork.
Serve
Once cooked, the chicken can be served whole or shredded, depending on preference. Ladle the tender chicken and rich gravy over white rice or mashed potatoes for a complete comfort meal.