Fill a medium pot halfway with cold water and add sausages. Bring to a boil and shut off the flame. Let sausages sit in water until water has cooled to room temperature. Remove sausages from water and reserve until ready to grill.
For the peppers and onions: Halve the cherry tomatoes. Seed and slice the peppers into julienne, peel and slice the onions, soup style. Thinly slice the serrano peppers and the garlic cloves. In a large mixing bowl toss all of the vegetables together with the balsamic vinegar, olive oil, salt and pepper. Pour the mix onto a large oven tray and bake for about 1 hour, stirring occasionally so that it cooks evenly. Remove from oven when vegetables are fully cooked and lightly browned. Let cool slightly and stir in chopped basil.
Grill the poached sausages over a medium-high heat until nicely caramelized