Step 1: Arrange and Season the Chicken - Place the four chicken breasts in the bottom of a 6 quart slow cooker in as even a layer as possible. Sprinkle the seasoned salt, black pepper, garlic powder, and onion powder evenly over the chicken. I like seasoning the chicken directly instead of relying only on the sauce because it gives every piece a better base of flavor. If your chicken breasts are especially large, you can cut them horizontally into smaller portions so they cook more evenly.
Step 2: Whisk Together the Creamy Gravy - In a medium bowl, combine the cream of chicken soup, cream of mushroom soup, chicken broth, Worcestershire sauce, and poultry seasoning. Whisk until the mixture is smooth and the condensed soups are fully blended with the broth. It will look a little loose at this stage, and that is exactly what you want because some of that moisture will cook into the chicken and the gravy will thicken later.
Step 3: Add the Mushrooms and Butter - Pour the creamy soup mixture evenly over the seasoned chicken breasts. Scatter the sliced mushrooms across the top, then cut the two tablespoons of butter into small pieces and place them over the mushrooms. As the butter melts, it works its way through the mushrooms and sauce and adds an extra bit of richness without requiring you to stir anything halfway through cooking.
Step 4: Slow Cook Until the Chicken Is Tender - Cover the slow cooker and cook on LOW for 4 to 6 hours or on HIGH for about 2½ to 3½ hours. The exact timing depends on your slow cooker and the thickness of your chicken breasts, so start checking near the beginning of the range. The chicken is ready when the center reaches 165°F and the meat is tender. I would rather check a little early than let chicken breasts stay in there too long and dry out.
Step 5: Stir and Settle the Mushroom Gravy - Once the chicken is fully cooked, gently stir the gravy and mushrooms around the chicken breasts. Keep the chicken whole instead of shredding it. That makes the plate feel much more like the restaurant version, and there is something extra satisfying about cutting into a tender chicken breast with a fork while the gravy runs down into the rice. If the sauce looks a little thin when you first lift the lid, give it a few minutes. It thickens as it stands.
Step 6: Simmer the Seasoned Rice - About 30 minutes before you plan to eat, add the long grain white rice, chicken broth, poultry seasoning, garlic powder, and kosher salt to a medium saucepan. Bring the mixture to a boil over medium to high heat, then immediately reduce the heat to low and cover the pan. Let the rice simmer for 18 to 20 minutes, or until the grains are tender and the broth has been absorbed. Try not to keep lifting the lid because that trapped steam is doing important work.
Step 7: Rest and Butter the Rice - Remove the saucepan from the heat and leave it covered for 5 minutes. That short resting period helps the rice finish steaming so the grains stay fluffy instead of wet or sticky. Add the three tablespoons of butter, then gently fluff the rice with a fork until the butter melts through it. At our house, this is usually the moment somebody wanders into the kitchen asking when dinner will be ready.
Step 8: Build the Plates and Serve - Spoon a generous amount of buttery seasoned rice onto each plate. Set a whole chicken breast, or an appropriate portion of chicken, over the rice and spoon plenty of mushroom gravy across the top. Make sure everybody gets some of those tender mushrooms too. Add fresh parsley if you like a little color, then serve while everything is hot and the gravy is settling into all those fluffy grains of rice.