Preheat the oven to 200°C/400°F.
Cut the broccoli into bite-sized pieces.
In a baking tray add broccoli, garlic powder, olive oil, sea salt, and black pepper. Toss until combined.
Roast in the oven for 25-30 minutes until the broccoli florets are golden brown and crispy. While waiting for the broccoli to finish roasting, you can make the cheese sauce.
In a pan, melt butter over medium heat. After melted, add the flour and toast until the flour turns a little bit golden about 2-3 minutes.
Reduce heat down to low, add 1/4 part of the milk while mixing so there is no lump on the saucepan. Once blended, add the remaining milk in 1/4 part increments, mixing well after each addition, until all the milk is used and sauce is free from lumps.
After the sauce thickens, add black pepper, parmesan cheese, and cheddar cheese. Mix until the cheese is melted.
The sauce is ready to serve.
You can enjoy the roasted broccoli and drizzle the cheese sauce on top. Or you can put the cheese sauce in a bowl and enjoy the roasted broccoli by dipping it to the sauce.