For the Potatoes:

1. Preheat the oven to 425F. Prick the potatoes all over with a fork and rub the olive oil over the skins, rubbing with your hands to fully coat. Season generously with salt and pepper.
2. Place the potatoes on a baking sheet and place in the oven for 40-50 minutes, or until easily pierced with a fork.

For the Broccoli:

1. Spread the broccoli out in an even layer on another baking sheet. Drizzle with the oil and season with salt, pepper, and a pinch of red pepper flakes. Toss well to combine. Add to the oven with the potatoes after the potatoes have been baking for roughly 20-25 minutes.
2. Roast, stirring once or twice, for 20 minutes or until the broccoli is tender and charred in spots.

For the Cheese Sauce:

1. While the vegetables are roasting, make the cheese sauce by melting the butter in a medium saucepan. Add the onion and cook over medium-low heat for 5 minutes, or until the onions are translucent and tender.
2. Add the flour and toss to coat the onions evenly. Cook for 1-2 minutes, stirring frequently.
3. Slowly stream in the milk, stirring constantly, and cook over medium heat until the mixture thickens enough to coat the back of a spoon, about 5 minutes.
4. Add in the grated cheese and stir until it is melted. Season, to taste, with salt and pepper.

To Serve:

1. Cut the potatoes down the center lengthwise and fluff the inside a bit. Spoon some of the cheese sauce over the potato and then top with the roasted broccoli. Spoon more cheese over top and serve immediately, garnished with scallions or chives.