Canned white asparagus from Navarra is so revered in Spain that it has its own D.O. (a protected denomination of origin). This simple, composed salad is a lovely way to showcase its buttery flavor.
In a blender, puree the coarsely chopped tomato with the lemon juice, mustard and sugar. Add the mayonnaise and the 3 tablespoons of olive oil and blend the dressing until thick. Season with salt and pepper.
In a large saucepan of boiling salted water, cook the green beans until tender, 4 minutes. Drain and let cool to room temperature, then drizzle the beans lightly with olive oil and season lightly with salt and pepper.
Arrange the beans, tomato wedges, onion and asparagus on a platter. Drizzle the dressing over the vegetables and serve.
The dressing can be refrigerated overnight. Serve lightly chilled or at room temperature.