This is a quick and easy thing I invented yesterday, and it worked out perfectly. First, I steamed two yellow peppers until they were cooked (first removing the inner pith and seeds with a sharp knife) and then peeled off the skins, so that only the soft yellow flesh was left.
Next, I whizzed up half a red onion in the trusty Magimix, together with a red chili (though, of course you could also use a green one), and two cloves of garlic. Once they had all been thoroughly chopped up, I added the yellow pepper flesh, and using the "pulse" button combined the ingredients carefully. You don't want the peppers to break down too much. Then it was a simple matter of adding fresh lime juice, lots of Maldon Salt, and a good quality Extra Virgin Olive Oil.
It would work brilliantly with cold beef, and the yellow colour is interesting. The sweetness of the peppers balances out well with the sharpness of the lime. Try it, and I don't think you will be disappointed.