Preheat oven to 400° Fahrenheit and line a large baking sheet with parchment paper.
Prepare Italian sausage, peppers and onion as noted and add them to the sheet pan. Add mushrooms to the sheet pan (cut them into even sizes if needed).
Add avocado oil, balsamic vinegar, garlic, Italian seasoning, salt and pepper to a small dish or bowl and mix. Pour the mixture over the sausage and veggies on the sheet pan and toss to coat. Spread everything out in a single layer as evenly as possible.
Place in the oven to cook for 20-25 minutes, or until the veggies are cooked to your liking.
While the sausage and peppers are cooking, make the sauce (optional).
Place all of the sauce ingredients in a pint jar or a 1 or 2 cup pyrex measuring cup. You want a container just wide enough for your immersion blender.
Place the immersion blender at the bottom of the container. Start blending while holding the blender at the bottom. Don’t move the blender from the bottom until the sauce begins to emulsify and thicken. Once it starts to thicken, slowly move the immersion blender to the top until all of the oil is completely mixed in. It should result in a medium-thick sauce (slightly thinner than mayo).
When the sausage and peppers are cooked, remove and serve with sauce (optional) and sliced basil (optional).
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