Preheat oven to 425°F and line a sheet pan with parchment paper.
In a small bowl, whisk together the mustard, honey, olive oil, thyme, salt, and pepper.
Arrange sausages, bell peppers, and onion wedges on the sheet pan with space between each piece.
Brush half of the honey mustard mixture over everything, making sure to coat the sausages well.
Roast for 15 minutes, then flip the sausages and brush with remaining honey mustard mixture.
Continue roasting for 10 more minutes until sausages are browned and vegetables are caramelized.
Serve immediately with crusty bread if desired.