Preheat the oven to 200C/180C fan.
Peel the red onions and chop them roughly into sixths, slice the peppers into strips, dice the potato into approx 1" chunks. Peel, de-core and slice the apple.
Add all the veg to a large baking tray along with the garlic cloves (whole, skin on), thyme sprigs and the sausages.
In a small bowl mix together the olive oil, balsamic vinegar and honey. Drizzle this all over the sausages and veg then sprinkle over some salt and pepper to taste.
Toss to coat everything in the glaze then put the sausages back on top (to help them brown).
Bake for 25 minutes then toss everything once more/turn the sausages and bake for a further 20-25 minutes or until everything is sticky and golden brown.
Carefully peel the garlic and discard the skins. You can then break the roasted garlic apart and distribute it around the tray more if desired.