Bring a large pot of water to a boil.
Add the broccoli and blanch 5-6 minutes.
Rinse with cold water to stop the cooking.
Drain well and set aside.
Preheat oven to 350°.
Grease a 9 x 13 inch baking dish with cooking spray.
Rub the chicken with olive oil and season with sea salt and pepper.
Bake at 350 degrees for 20 minutes (I love to use a clay roaster for this because it keeps the chicken very moist.
Cool and transfer chicken to a cutting board.
When cooled, slice chicken into bite sized pieces.
Heat a skillet on medium heat.
Add the butter and oil until melted and then add garlic and onion and cook until soft.
Sprinkle the flour and whisk until smooth.
Stir in broth, milk and sherry and bring to a boil while whisking.
Remove from heat and stir in half of the cheddar cheese.
Season to taste with salt and pepper.
Arrange the broccoli in a casserole dish and pour half of the sauce over the broccoli.
Arrange the chicken on top and cover with remaining sauce. Sprinkle the remaining cheddar cheese, grated parmesan and top with breadcrumbs.
Spray a little oil on top.
Bake 30 minutes, serve hot.