Either roast the pepper untill the skins are black, or skewer over a gas flame (or bbq) until skins ditto, then put in a plastic bag to steam until cool enouigh to handle. Strip the skins off, then chop. Sweat an onion in oil/butter until soft. Add peppers and either a tin of chopped plum tomatoes, or some peeled fresh (well-ripe) ones. (Nick a cross in the skin of toms, pour over boiling water. The skins should peel themselves off, just about) Add some stock. Boil, then simmer until toms are soft. Blitz in a food-processor, sieve if you like. Thin to required consistancy, with water/stock/tomato juice. season.
No recipe and this is untried but I would: Cut onions, peppers, garlic into chunks and roast in olive oil with some salt and pepper and some smoked paprika, maybe a few chilli flakes, adding tomatoes about half way through.
Blitz, thinning with vege stock if needed. Warm through, maybe with a bit of double cream.