Add the onion, oil, and salt to a small skillet. Turn the heat to medium-low and cook for about 1 hour, stirring every 10 minutes, until the onions are very soft and golden in color but not browned at the edges at all. The onions shouldn't sizzle, just gook very gently, so turn the heat up or down as you go.

When the onions are cooked, combine them with the yogurt, feta, garlic, and basil leaves. Stir together and taste; add as much salt as you'd like. Let the dip chill for 15 minutes or more in the fridge. Serve topped with the tomato with chips or crackers for dipping.

Enjoy!