I like foods that add a touch of luxury, where a little goes a long way. I'm thinking of things like caviar, smoked fish, truffles, whipped cream, chocolate. Nuts fall into that category for me too, maybe it's their association with the holidays or with desserts like cakes and cookies. Or maybe it's because they are so rich.
Walnuts have a richness due to their fat content. They have an inherent sweetness but also a slight bitterness. That bitterness is actually what complements so many foods. The flavor of walnuts is more mellow and buttery when toasted which is great for baked goods and desserts. But when it comes to strong foods like beef, bitter greens, cheese and herbs like basil, un-toasted walnuts add another more complex dimension. If you've made pesto you might have noticed that most pesto recipes call for un-toasted nuts, so clearly I'm not the first to realize this.
In experimenting with different kinds of nuts, I have found walnuts to be the most versatile of all due to their buttery, rich, sweet and bitter flavors. I also found that lemon almost always complements walnuts, toasted or not. The possibilities are endless, salad topped with walnuts and tossed with a lemon dressing, lemon tea cake with walnuts, lemon walnut biscotti, lemon peel and walnuts as a topping for green beans...