Coat a grill or grill pan with cooking spray; preheat to medium-high.
In a large mixing bowl, combine turkey, onion, garlic, fennel seed, salt and pepper; form into four 1/2-inch-thick patties. Coat burgers and inside of rolls with cooking spray.
Grill burgers for 5 minutes; flip burgers and add rolls to grill. Grill open-face rolls until lightly charred, about 1 to 2 minutes; remove to serving plates. Grill burgers until cooked through, about 5 minutes more.
To serve, top grilled rolls with burgers and tomato slices; garnish with basil. Yields 1 burger per serving.