Slice the onion into thin strips and sauté in a pan over medium-high heat until lightly browned.
Crack the eggs into a bowl and season with salt and pepper. Chop the chives and basil and fold into the eggs.
When the onions are brown, pour the eggs and cook until you see the edges turn opaque.
Top the frittata with the crumbled feta and halved cherry tomatoes. Place under the broiler and cook for 5 to 7 minutes or until the top is cooked and golden.