Preheat oven to 450°F/205°C.
Rub peppers with olive oil and place on a cookie sheet. Bake peppers in the oven for about 20 minutes or until the skins have darkened and begin to bubble. Remove peppers from the oven and place in a bowl. Cover bowl with plastic wrap and allow peppers to soften for 10 minutes.
Carefully remove warm peppers from the bowl - they will still be quite warm. Remove the skin and seeds. Slice the remaining flesh into long strips.
Heat a 10-in. omelette or fry pan over high heat. Add 2 Tbsp. olive oil to coat the pan. Add sliced onions and sauté for 2 - 3 minutes, until softened. Add the peppers and toasted seeds and continue to sauté another 2 minutes. Turn off the heat and add in the red wine vinegar. Adjust seasoning to taste.
Place the watercress and coriander in a separate bowl. To serve, toss the warmed ingredients with the greens.