Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper.
Toss the trimmed and halved Brussels sprouts with olive oil, salt, and black pepper directly on the baking sheet. Arrange them cut-side down for better caramelization.
Roast for 20-25 minutes, or until the sprouts are tender and the edges are crispy and golden brown.
While the sprouts roast, place the diced beef bacon in a large skillet over medium heat. Cook, stirring occasionally, until the fat has rendered and the beef pieces are dark and crispy, about 6-8 minutes. Remove the beef with a slotted spoon and set aside on a paper towel, keeping about 1 tablespoon of the rendered fat in the pan.
In the same skillet with the reserved beef fat, reduce heat to low. Add the minced garlic and sauté for 30 seconds until fragrant.
Whisk in the heavy cream, Dijon mustard, and whole grain mustard. Simmer gently for 2-3 minutes until the sauce slightly thickens.
Remove the roasted Brussels sprouts from the oven and transfer them into the skillet with the sauce. Toss gently to coat.
Top with the crispy beef pieces and serve immediately.