Roast the red peppers directly over a flame on the stove or on the hottest part of a grill, turning, until they are completely charred and soft. Let cool, then peel and remove the cores and seeds. Slice thinly, then add to a bowl.
Shave the snow peas, summer squash and celery root as thinly as you can using a knife, mandoline or vegetable peeler (the snow peas need to be done with a knife), then add them to the bowl with the peppers. Season everything with the red wine vinegar, oregano and some salt and toss well to coat. Roughly chop the basil and add it along with the olive oil. Allow to marinate 1 hour before serving. Best served alongside anything.