Heat olive oil in a pan over medium heat.

Add diced onion to the pan and saute until tender and translucent.

Remove the pan from heat and let the onions cool slightly.

In a bowl, combine ground turkey with the sauteed onions and any oil from the pan.

Mix well until all ingredients are evenly distributed.

Form the mixture into 4 equal-sized patties.

Preheat grill or heat a pan over medium heat.

Cook patties for about 5 minutes on each side, or until cooked through and no longer pink inside.

While the patties are cooking, split the hamburger buns and toast the cut sides.

Once the patties are cooked, place them on the bottom buns.

Top with lettuce and tomato slices.

Add the top bun and serve immediately.