Preheat a grill to medium-high heat or an oven to 400°F. While the heat builds, slice the peppers into strips and the onion into half-moons.

In a large bowl, toss the peppers and onion with the olive oil, Italian seasoning, garlic, salt, black pepper, and red pepper flakes if using. If using mushrooms, stir them into the mixture.

Cut four large sheets of heavy-duty aluminum foil and divide the vegetable mixture among them. Lay the sliced Italian sausage over the top of each pile of vegetables. If using balsamic vinegar, drizzle it evenly over the packets.

Fold the foil into tightly sealed packets and cook on the grill or on a baking sheet in the oven for about 20 minutes, until the sausage is cooked through. Let the packets rest briefly, then carefully open one corner away from your face to release the steam before serving.