Preheat the grill to medium-high.
Toss the vegetables with the olive oil and grill until the vegetables are lightly charred and tender. Peel the blackened skin off the peppers and discard. Season the vegetables and set aside.
Preheat the oven to 180°C/350°F/Gas 5.
Slice the salmon fillets carefully horizontally in half. Lay on a baking sheet, pressing down very gently on each salmon piece with your hand to flatten it a little. Season and bake in the oven for 5 minutes or until just cooked.
Layer the vegetables and salmon alternately in a shallow-sided dish to create a colourful layered effect. Cover with foil and put weights such as small cans or ramekins on top of the foil to press down on the mille feuille.
Chill for two to three hours.
To serve: toss the chopped tomatoes with the oil and vinegar and a little seasoning. Spoon equal portions on to plates. Carefully cut out either circles or squares of the mille feuille and place alongside, drizzled with a little extra virgin olive oil if desired.