Preheat the oven to 425 and lightly oil a large baking sheet or line it with parchment paper or aluminum foil for easy clean up.
Remove stem and seeds from the peppers and cut them into slices.
Slice the red onion into thin slices and finely mince the garlic cloves.
Place peppers, onions and grape tomatoes in a big bowl. Add in minced garlic, salt and pepper. Drizzle olive oil over the vegetables and stir to coat them evenly.
Pierce sausages a few times with a knife or fork and place on the prepared baking sheet.
Spread the vegetables evenly around the sausages in a single layer.
Place the baking sheet in the oven and bake for 20 minutes.
Flip the sausages over and gently stir the vegetables.
Return the pan to the oven for another 15 minutes.
Serve warm, store any leftovers refrigerated in an airtight container.