Preheat the oven to 450. Set the oven rack in the top ⅓ of the oven. Line a baking sheet with parchment paper and set aside.
Toss the onion, bell pepper, and potatoes with the avocado oil, salt, and pepper. Scatter them on the prepared sheet pan in a single layer.
Add the sausages to the sheet pan. Prick them a few times with a fork or knife. Bake in the preheated oven for 30 minutes.
While the sausage and veggies bake, make the honey mustard sauce by adding the honey, dijon, olive oil, grainy mustard, apple cider vinegar, garlic, and salt to a small bowl. Stir until combined.
Remove the sausage and veggies from the oven. Baste with about ⅔ of the sauce. Return the sheet pan to the oven for another 10 minutes.
Remove from the oven and serve with the remaining sauce.
