For sauce: In a small bowl, stir mayonnaise, ketchup, and Onion & Garlic Tableside Sprinkle together until combined. Reserve for serving.
For tostones: In a large Dutch oven or heavy-bottomed pot over medium heat, bring oil to 320 degrees. Meanwhile, prepare plantains by cutting off both ends and scoring the peel down the length of plantains on 4 sides. Try to only cut through the peel and not into the plantain. Peel plantains and slice into 1-inch pieces. In batches, add plantains to oil and fry for 3 min. or until light golden brown. Use the bottom of a glass or measuring cup to smash each piece of fried plantain to create thick coins, about 1/4 to 1/3-inch thick. Increase temperature of oil to 375 degrees and add smashed plantains to oil, frying for another 90 seconds to 2 min. or until deep golden brown. Sprinkle with French Fleur de Sel and serve immediately with reserved sauce.