In a large non-stick skillet, heat oil & add onion. Cook on medium-low heat for 10 minutes. Season with a little salt, stir & continue cooking, uncovered, for another 20 minutes (30 minutes total), stirring every five to ten minutes. You will want to caramelize the onions LOW & SLOW. If they look dry at all, add a Tablespoon or two of water occasionally. If the onions start to stick to your pan, add a little more olive oil. Keep warm.
Cook turkey burgers according to the directions on the package: Preheat grill to medium heat. Lightly spray or brush both sides of FROZEN turkey burgers with oil and place on grill, about 4" above heat. Grill burgers 8 minutes on one side; turn and grill on the other side for 7 minutes or until center of burger reaches 165 degrees Fahrenheit.
Top warm burgers with one slice of swiss cheese and a scoop of warm caramelized onions. Place on a toasted hamburger bun. Enjoy!