How to Make It
In a food processor, pulse 1/2 cup of the walnuts with the basil and garlic until finely chopped. Add the oil, mayonnaise and lemon juice and process until smooth. Season with salt and pepper.
Coarsely chop the remaining 1/4 cup of walnuts. In a large bowl, toss the fennel with the cabbage, bresaola, chopped walnuts and pesto. Transfer to plates and serve.