Preheat oven to 350° F. Lightly coat a casserole dish with nonstick cooking spray (I used a 500 mL dish).
Lightly coat a small cast iron (or whatever kind you like) skillet with olive oil. Heat over medium heat and add onion. Cook, stirring frequently, until lightly browned (for a couple minutes). Transfer to a heatproof bowl. Add drained spinach to the onion and stir in mayonnaise. Crumble in 3 ounces Swiss cheese and mix with the other ingredients. Spoon into prepared dish and top with remaining 1 ounce Lorraine® cheese. Bake 15 minutes, until heated through and the cheese is melted and bubbly.
Serve! Use chips, veggies, bread, etc. for dipping.