Steam the broccoli, just until fork tender. Set aside.
Cook the tortellini according to the package directions. Reserve 1/4 cup of the pasta water, and drain the rest. Set aside.
While the pasta's cooking, begin the sauce by melting the butter in a large skillet over medium heat.
Stir in the onion, and let it cook until tender. Stir in the garlic and red pepper flakes, cooking for roughly two minutes or until fragrant.
Stir the half & half into the skillet, and allow the mixture to come to a simmer. Allow the mixture to simmer for 3-4 minutes, stirring occasionally.
Reduce the heat to low, and stir in the Parmesan cheese. Stir until the cheese is melted.
Add the reserved pasta water, and stir until it's incorporated into the sauce. Allow the sauce to thicken, stirring as needed.
Add the cooked pasta & broccoli to the skillet, tossing gently to coat.
Season with salt & pepper, to taste, and serve. Top with extra shredded Parmesan cheese, if desired.