Preheat the oven to 425°F (220°C) and line a rimmed baking sheet with foil.
Toss the halved Brussels sprouts with olive oil, salt, and black pepper, then spread them cut-side down in a single layer on the prepared baking sheet.
Roast for 20 to 22 minutes, until the cut sides are deeply golden and the outer leaves are slightly crisped. Do not stir during roasting.
While the sprouts roast, cook the bacon pieces in a large skillet over medium heat for 4 to 5 minutes until crispy, then remove to a paper towel-lined plate, leaving the fat in the pan.
Add the summer sausage rounds to the skillet and cook for 2 minutes per side until the edges are browned and slightly caramelized. Remove and set aside with the bacon.
Add the sliced onion to the same skillet and cook over medium heat for 5 to 6 minutes, stirring occasionally, until softened and beginning to caramelize.
Add the garlic and cook for 1 minute, stirring.
Stir in the whole grain mustard, apple cider vinegar, brown sugar, and smoked paprika, mixing until the onions are coated.
Add the roasted Brussels sprouts, sausage, and bacon to the skillet and toss everything together over medium heat for 1 to 2 minutes until warmed through and well combined.
Taste and adjust seasoning, then transfer to a serving dish or serve directly from the skillet.