Preheat the oven to 350 degrees.
Steam the broccoli for a few minutes until crisp tender.
Cook the onions in butter in a skillet over medium heat on the stove until they are tender.
Add the minced garlic, and saute another minute.
Stir in the heavy whipping cream, and simmer until beginning to thicken.
Mix in the sour cream, parmesan cheese, lemon juice, thyme and salt and pepper to taste.
Place 1 cup of cheddar cheese, the shredded chicken and the broccoli into the skillet.
Stir until everything is well combined.
Layer the mixture in a well greased 9x9 baking dish.
Top with the remaining cheddar cheese and bake for 20 minutes.