Preheat oven to 350F. Slice leek into 3/4" rings. Place the leeks into a medium bowl filled with water. Gently swish the leeks around in the water to dislodge any dirt or sand that is stuck in the leek layers.
Remove leeks from the water and spread out on a baking sheet lined with a silicone pad or tin foil. Drizzle with olive oil and sprinkle with salt and pepper. Toss to coat.
Place the leeks on the middle rack of the oven to roast while the risotto is cooking.
Place the stock in a pot and heat over medium low. You want the stock to be warm when adding it to the risotto so that the temperature change does not affect the rice.
In a large pot over medium heat, melt 3 Tbsp of the butter. When melted, add the red onion. Cook until the onion is translucent but not yet browning, approximately 3-5 minutes.
Add the rice and stir. Continue cooking to slightly toast the rice until it is just starting to show a little brown color, approximately 3-5 minutes.
Add the white wine to the pot. Cook until the rice is almost dry, where most of the wine has been absorbed.
Add one cup of stock to the rice. Stir continuously until most of the stock has been absorbed. Repeat this process one cup of stock at a time until the rice is cooked thoroughly. You want the rice to have just a little bit of bite left so that it is not completely mushy. Depending on your rice, you may need more or less stock for the right consistency so start tasting the rice after 4 cups have been added.
When your rice is cooked through, remove the pot from the heat. Add the rest of the butter, the Parmesan cheese, roasted leeks, and brie. It is important to do this before adding salt and pepper because the cheese is salty and the leeks have salt and pepper on it. Stir to combine.
Taste the risotto and season with salt and pepper as desired.