If using fresh, raw beans, bring a large pot of water to boil, add beans, return water to boil and boil for 3 minutes or until done to desired tenderness.
Cool beans in ice water to prevent continued cooking. Drain well.
Toss beans with tomatoes, onions, basil and feta cheese.
In a separate small jar, mix oil, lemon juice, mustard, cayenne pepper, salt and black pepper. Shake vigorously.
Pour on top of bean mix.
Serve immediately or store in fridge for up to 24 hours to let flavours blend. This salad will last up to 2 days in the fridge.
Notes
You could use frozen or canned green beans - no need to boil in water, simply thaw frozen beans and drain canned beans.