Preheat your grill to medium-high heat (around 375-400°F).
Slice the peppers and onions into 1/4-inch strips. Try to keep them uniform in size for even cooking.
In a large bowl, toss the peppers and onions with olive oil, minced garlic, Italian seasoning, salt, and pepper.
Create a foil packet with heavy-duty aluminum foil, or use a grill basket for the vegetables.
Place the sausages on the grill and close the lid. Cook for 4-5 minutes per side.
Add the vegetable packet to the grill. Cook for 12-15 minutes, stirring occasionally, until the vegetables are tender-crisp.
Move the sausages to indirect heat if they're browning too quickly.
Check that sausages reach an internal temperature of 160°F before serving.