While the chicken cooks, melt the butter in a large sauté pan.
Add in the onions and sauté until softened and slightly caramelized.
Add the garlic to the pan and cook for one minute, or until garlic is very lightly golden.
Pour the chicken broth into the pan and bring to a boil. Let simmer for 10 minutes or until slightly reduced and thickened.
Add the thyme, lemon zest and lemon juice to the pan, then stir in the heavy cream and parmesan until evenly combined.
Pour the sauce over the chicken and serve with rice, pasta, or potatoes.