Preheat the oven to 400 degrees F. Spray two 12-cup muffin tins with non-stick cooking spray, every other cup. Meanwhile, place 2 cups chicken stock and rice in a sauce pot. Bring to a boil, cover, and reduce the heat to a simmer. Cook the rice for 15-20 minutes, until the stock absorbs and air holes form on the top. Fluff the rice with a fork and set aside.
Cut each puff pastry sheet into 4 equal squares and place a square down into the sprayed muffin cups, leaving the points up.
While the rice is cooking, melt the butter in a large skillet. Add the onion and broccoli to the skillet. Saute for 3-4 minutes until the broccoli is vibrant green and slightly soft. Add the cooked rice to the broccoli, then add 1 1/2 cups of shredded cheese. Stir to combine. Then add 1 cup of chicken stock and 1/4 cups heavy cream. Stir until the cheese has melted to form a sauce.
Spoon the broccoli rice filling into the puff pastry cups. Sprinkle the tops with the remaining cheese and bake for 20-25 minutes, until golden brown. Cool in the pan for 5 minutes, then gently lift out. Serve warm or at room temperature.