Melt butter in a large, heavy pot over medium-high heat.

Add the finely diced onion to the pot.

Sweat the onions until softened and translucent.

Stir in the all-purpose flour until it is fully incorporated with the onions and butter.

Gradually whisk in the chicken stock, ensuring no lumps form.

Add the frozen broccoli to the pot.

Bring the soup to a simmer.

Cook for 20 minutes, allowing the broccoli to become tender.

Stir in the heavy cream.

Using an immersion blender, roughly puree the soup until smooth.

Season with salt and freshly ground black pepper to taste.

Serve hot.