Preheat your oven to 450°F. Line a baking dish with foil and grease the foil. Arrange the chicken breasts in the prepared pan. Spray them with oil and sprinkle with salt and pepper. Bake until cooked through, about 20 minutes.
Meanwhile, grease a 2-quart casserole dish. In a large bowl, mix the mayonnaise, sour cream, Dijon mustard, minced garlic, salt, pepper, dried thyme, paprika, cayenne, and ½ cup of shredded cheddar.
When the chicken is cooked, remove it from the oven but keep the oven on. Allow the chicken to slightly cool until it’s easier to handle. Cut it into cubes or shred it. Add the chicken cubes to the bowl with the mayonnaise-sour cream mixture and toss to coat.
Transfer the mixture to the prepared casserole dish. Sprinkle with the remaining ½ cup of shredded cheddar. Bake until the cheese is melted, 5-10 minutes. If your casserole dish is broiler-safe, you can broil it instead, 6 inches below the broiler element (not directly below), for 30 seconds to 1 minute.
Sprinkle the casserole with sliced scallions and bacon bits. Serve immediately.
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Notes
If you'd like, you can very lightly steam small broccoli florets and mix them into the casserole before the final stage of baking, making it a complete meal. If you do, I suggest you increase the mayonnaise and sour cream to ⅓ cup each.
You can keep the leftovers in the fridge, in a sealed container, for 3-4 days. Gently reheat them, covered, in the microwave at 50% power. If you used pre-cooked chicken, take that into account when counting those 3-4 days.