Season the pork medallions on both sides with salt, pepper, and thyme.
Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Add the pork medallions and cook for 3 minutes per side until browned and just cooked through. Remove them to a plate.
Lower the heat to medium and add the remaining tablespoon of olive oil to the skillet. Stir in the garlic and cook for 30 seconds until fragrant.
Add the Brussels sprouts to the skillet. Cook for 6 to 8 minutes, stirring occasionally, until they are tender with some browned edges.
Stir the Dijon mustard into the Brussels sprouts until evenly coated. Return the pork medallions to the skillet and toss everything together for 1 minute to warm through and combine flavors. Serve right away.