Heat oven to 375° F.
Spread the broccoli florets on a baking sheet, drizzle with a bit of olive oil, a pinch or two of sea salt, and a few grinds of black pepper. Roast for 15 - 20 minutes.
Meanwhile, warm the oil in a large sauté pan over medium-high heat.
Add the onion and sauté for 3-5 minutes, stirring occasionally. Add the mushrooms and garlic and sauté for another 5 minutes or so, stirring occasionally, until the mushrooms are cooked through.
Sprinkle the flour evenly over the onion/mushroom mix. Cook for a minute, stirring occasionally so the flour will blend in. Add the stock, and stir until most of the clumps are gone.
Sprinkle the flour evenly over the onion mixture and stir until combined. Cook for 1 minute, stirring occasionally. Add in the stock, and stir everything together until most of the clumps are gone.
Add the milk, Dijon, herbs, a strong pinch of sea salt, and a few grinds of black pepper. Stir until combined.
Lower the heat and continue cooking until the sauce has reached a low simmer. Remove from heat and stir in 1 ½ cups of the shredded cheese. Taste and season with additional salt and pepper, if needed.
In a large 9 x 13-inch baking dish, combine the rice, broccoli, cheesy mushroom sauce, and chicken. Smooth the casserole out into an even layer.
Bake uncovered for 15-20 minutes
Remove the baking dish from the oven, sprinkle the remaining cheddar cheese evenly on top of the casserole, and bake for 10 -15 minutes or until the cheese is nice and melty.
Serve warm, garnished with extra black pepper and/or fresh herbs, if desired.