For the salad, place the cherry tomato halves into a bowl. Strip the corn kernels off the cob using a sharp knife and place the corn into the bowl with the tomatoes. Add the chopped spring onions, olive oil and crabmeat, season, to taste, with salt and freshly ground black pepper and stir together. Spoon onto a serving plate and top with the roasted tomatoes.
For the mayonnaise, place the egg yolks, mustard and white wine vinegar into a bowl and whisk until combined. Gradually add the olive oil in a thin stream, whisking continuously until the mixture is thickened and emulsified. Add a little lemon juice, to taste, and stir in the chives.
To serve, place a dollop of mayonnaise on top of the salad.