Preheat oven to 450°. Bring a large pot of water to boil and add wax beans. After 1 minute, add the green beans. Cook 4 minutes; drain and rinse with cold water. Spread beans on a dish towel and pat them dry. Put in a large serving bowl and set aside.
Toss the peppers with a tablespoon of oil and spread on a baking sheet. Bake 5 minutes. Remove from oven and add to bowl with the beans.
Heat 3 tablespoons of oil in a small pan over medium heat. Add garlic; stir to coat with oil. Quickly add the capers, cumin and coriander. Do not let the garlic burn — stir frequently. After about a minute, the garlic should just be turning golden — remove from heat and scrape the contents over the beans and peppers. Toss gently to mix, and add scallions, herbs and zest. Season with salt and pepper.
Keeps refrigerated for up to a week, but let it come to room temperature before serving. Serves 8 as a side dish or part of a mezze table.