1. Preheat the oven to 400°F. Add the potatoes to a medium pot and cover with cold water by about 1 inch. Season with 1 teaspoon salt and bring to a boil. Lower to a simmer and cook until just fork-tender, about 7–8 minutes. Place broccoli in a colander and drain the potatoes over it so the broccoli lightly steams.

2. In a small bowl, combine onion powder, garlic powder, dill, and ½ teaspoon black pepper. In a large bowl, toss the chicken with 1 tablespoon of the seasoning blend and 1 teaspoon salt until evenly coated.

3. In a large oven-safe pot or Dutch oven over medium-high heat, melt 1 tablespoon butter. Add the breadcrumbs and 1 teaspoon of the seasoning mix. Stir constantly until golden and toasted, about 2–3 minutes. Transfer to a plate and set aside.

4. In the same pot, melt the remaining 3 tablespoons butter. Add the chopped onion and cook until soft and translucent, about 6–7 minutes. Stir in the chicken and garlic and cook until the chicken is no longer pink on the outside, about 5 minutes. Sprinkle flour over the mixture and stir well. Slowly pour in the chicken broth, stirring constantly, and bring to a gentle simmer. Cook until the sauce thickens, about 3–5 minutes.

5. Reduce heat to low and stir in the cream cheese and another teaspoon of the seasoning mix until smooth. Add the potatoes and broccoli, gently stirring to coat everything in the sauce. Remove from heat and mix in ¼ cup sour cream and 1 teaspoon vinegar. Season to taste with salt and pepper. Transfer to a 9×13-inch baking dish. Sprinkle cheddar cheese evenly over the top, followed by the toasted breadcrumbs.

6. Bake uncovered until the casserole is bubbling around the edges and the cheese is fully melted, 15–20 minutes.

7. In a small bowl, whisk together the remaining ½ cup sour cream, remaining seasoning blend, and 1 teaspoon vinegar. Thin with a few teaspoons of water until pourable. Season with salt and pepper. Drizzle over the casserole or serve on the side.