Thoroughly combine the olive oil, vinegars, mustard, sugar and seasoning (try shaking in a lidded clean jam jar). Break the walnut halves into 2 pieces each, and halve, stone and peel the avocados. Slice each avocado half into thin diagonal slices then halve crossways.
Toss the leaves with half the dressing then divide the leaves between four serving plates. Arrange the avocado slices and nuts over the salad leaves and drizzle over the remaining dressing. Sprinkle the croutons on and serve immediately.
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