Preheat the oven to 350°F.
Grease an 8x12-inch baking dish with nonstick cooking spray or butter.
Heat the olive oil in a skillet over medium-high heat.
Add the chicken breasts to the skillet.
Cook the chicken for 3 to 4 minutes on one side.
Turn the chicken and cook for an additional 2 to 3 minutes on the other side.
Season the chicken with salt and black pepper.
Add approximately 1/2 inch of water to the skillet.
Reduce the heat to low.
Cover the skillet and cook for 5 to 8 minutes, or until the chicken is fully cooked.
Remove the chicken from the skillet.
Allow it to cool for 10 minutes.
Cut the cooled chicken into cubes.
Place the cubed chicken in the prepared baking dish.
Add the chopped celery, chopped onion, toasted sunflower seeds, cream of chicken soup, cottage cheese, lemon juice, dried parsley, and poultry seasoning.
Toss gently until the ingredients are evenly combined.
Season with additional salt and black pepper, if needed.
Spread the mixture evenly in the baking dish.
Place the chopped broccoli evenly over the chicken mixture.
Sprinkle the shredded cheddar cheese over the broccoli.
Scatter the crushed butter crackers evenly over the cheese.
Pour the melted butter evenly over the cracker crumbs.
Bake for 40 minutes, or until the cheese is fully melted, the casserole is heated through, and the cracker topping is lightly browned.
Remove the casserole from the oven.
Allow it to rest for several minutes before serving.
Serve warm.