Bring a large pot of salted water to a boil. Add the penne and cook according to package instructions until al dente. In the last 3 minutes, add the broccoli florets. Drain both and set aside.
Heat olive oil in a large skillet over medium heat. Add the chopped onion and cook until translucent, about 4-5 minutes. Stir in the minced garlic and sauté for 30 seconds until fragrant.
Lower heat to medium-low. Add milk to the skillet. Slowly whisk in shredded cheddar cheese until melted and smooth. Season with salt, black pepper, and cayenne pepper. Add more milk if needed to reach desired consistency.
In a large mixing bowl, toss together the drained penne, broccoli florets, and cheese sauce until evenly coated.
Preheat oven to 350°F (175°C). Transfer pasta mixture to baking dish. Combine breadcrumbs and grated Parmesan in a small bowl, then sprinkle evenly over pasta. Bake uncovered for 20-25 minutes until topping is golden and crispy.
Let the bake cool for a few minutes before serving to allow the sauce to thicken. Enjoy!