Preheat your oven to 350°F, bring a large saucepan of water to a boil, and get a large casserole dish out and ready to use.

Once the water is boiling, add the peeled and chopped potatoes and cook for 5-6 minutes, or until they easily pierce with a fork.

Once cooked, remove them from the boiling water using a slotted spoon and place them in your large casserole dish along with the cooked chicken breast. Don't drain the boiling water!

Add the chopped broccoli florets to the boiling water and cook for 1 minute. Now, you can drain the water and then transfer the cooked florets to your casserole dish.

Return the saucepan to the stove and add the butter. Melt over medium-low heat.

Once melted, add the minced onion and cook until soft and fragrant, about 5 minutes.

Add the flour and stir the butter, onion, and flour into a paste. Cook for 1 minute.

Pour in the chicken broth and milk. Increase the heat to medium and whisk continuously until the mixture has thickened enough to coat the back of a spoon.

Once thickened, stir in 1 cup of shredded cheese until melted.

Turn off the heat and stir in the Dijon mustard, salt, pepper, garlic powder, and onion powder.

Pour the cheese sauce over the chicken, broccoli, and potatoes and stir until well combined.

Cover your casserole dish tightly with foil, transfer it into your preheated oven, and bake for 20 minutes.

While it's baking, combine the olive oil and panko breadcrumbs in a small bowl.

After 20 minutes, remove the casserole from the oven, uncover it, and sprinkle with the remaining shredded cheddar and panko breadcrumbs.

Return the casserole to the oven and bake uncovered for 10 minutes, or until hot and bubbly.

Serve and enjoy!