This salad is visually stunning. Charring and peeling the peppers involves a fair bit of work, but the good news is that all components can be prepared several days ahead of serving. Don't add the vinaigrette in advance of serving time, however, or the vibrancy of the colors will be lost.
Preheat the broiler and place the whole peppers on a tray close to the heat source. Alternately, char the peppers on your grill. Either way, cook until blackened, turning the peppers so that all sides blister and char. Cool the peppers until they can be handled comfortably.
Peel off the blackened skin and rinse under cold water. Dry the peppers, then cut in half and core. Slice lengthwise into thin, even-sized strips.
Top and tail the beans to equal lengths. Bring a pot of salted water to a boil and cook the beans for 2 minutes. Drain and cool quickly by refreshing the beans under very cold water or in a bowl of ice and water. When cool, drain again.
Thinly slice the onion and place in a large bowl. Add the beans and peppers.
To make the dressing, place the vinegar in a small bowl and add the oil in a thin stream, whisking constantly. Add the tarragon or cumin, and season to taste with salt and pepper.
Pour the vinaigrette onto the vegetables and toss to coat. Arrange decoratively on a platter and sprinkle with the sliced almonds.