Preheat the oven to 220°C, gas mark 7.
Toss the tomatillos, spring onions and oil on a rimmed baking tray and season with the salt and pepper.
Roast, tossing once, until the skins are blistered in spots and beginning to separate from the flesh, approx. 15-20 minutes. Let them cool.
Purée in a food processor until smooth, then transfer to a medium-sized bowl.
Stir in the mayonnaise and lemon juice and season. Cover and chill until ready to serve.