Toss the squash in the olive oil and season with a quarter-teaspoon of salt. Spread evenly on an oven tray lined with baking paper and roast for 15-20 minutes, until just caramelising around the edges, then leave to cool down.
Meanwhile, make the dressing. Put the mustard, lemon juice, vinegar, sugar, a teaspoon of salt and half a teaspoon of black pepper in a medium mixing bowl. Slowly pour in the oils, whisking as you go, until you get a thick dressing.
Put all the salad ingredients in a large mixing bowl, pour over the dressing and toss well. Taste and adjust the seasoning – you may need a touch more lemon juice or olive oil, depending on your cabbage. Serve at once.