Place the ground chicken and onion in a mixing bowl. Drop chunks of the Boursin cheese throughout the mixing bowl using 2 teaspoons. (This will ensure the Boursin in evenly distributed.) Mix thoroughly, form into 4 patties, and freeze. Preheat oven to 400 degrees and grease a cast iron grill pan with olive oil cooking spray. Heat the empty cast iron skillet for 20-30 minutes on the bottom rack of the oven. Carefully remove the skillet from the oven and place two of the frozen patties in the grill pan. Roast on the bottom rack of the oven for a total of 20 minutes, flipping the patties over halfway through the cooking time. Repeat with the remaining 2 patties. Serve with toppings on hamburger buns.