Start by trimming and halving your Brussels sprouts, then toss them in a bowl with the olive oil, salt, and pepper until every piece is coated. Spread them out in a single layer on your baking sheet so they roast instead of steam. Giving them room on the pan is really what makes the difference between soft sprouts and crispy, caramelized ones.
Preheat your oven to 425°F (220°C) and roast the sprouts for 18 to 22 minutes, flipping them halfway through. You're looking for deep golden-brown edges and a slightly charred look on the cut sides. This is where most of that nutty, almost smoky flavor comes from, so don't pull them too early.
While the sprouts roast, season your chicken breasts generously with salt and pepper on both sides. Grill or pan-sear them for 5 to 6 minutes per side over medium-high heat, until the internal temperature reaches 165°F (74°C). Let the chicken rest for a few minutes before slicing so the juices stay locked in instead of running out onto the board.
In your small saucepan over low heat, combine the light cream or Greek yogurt, cracked pepper, Dijon mustard, and garlic if using. Simmer gently for 2 to 3 minutes, stirring often, until it thickens slightly and coats the back of a spoon. Keep the heat low here, since cream and yogurt can split if it gets too hot too fast.
If your rice isn't already made, cook it according to the package instructions while everything else is happening. I like to fluff it with a fork right before serving so it stays light instead of clumping together. A warm bed of rice really is what makes this dish feel like a full, satisfying meal.
Spoon a portion of rice onto each plate, then slice the rested chicken and fan it out over the rice. Add the roasted Brussels sprouts alongside, then generously spoon the creamy pepper sauce over the chicken. Serve immediately while everything is still warm and the sauce is at its silkiest.