Mix the dry package of Idahoan Roasted Garlic Potatoes with the butter until blended. Slice the French bread length wise and generously spread with the roasted garlic mixture. Toast under the broiler until butter spread is melted and toasted on top. (You can also grill the Garlic Bread--see footnote.)
Greek Salad: Drain onion from ice water and pat dry with paper towels. In a large bowl combine all salad ingredients, except cheese. Top the French bread with the Greek salad and sprinkle cheese over top. Serve with Tzatziki sauce.
Tzatziki Sauce: In a medium bowl, combine Greek yogurt, cucumber, lemon juice, garlic, and dill. Stir until well combined. Taste and season with salt and pepper.
Spread a small amount of the butter mixture on each slice of the French bread. Place face down on the grill until grill marks appear. Turn over and move to the indirect heat side of the grill and spread generously with more of the butter mixture. Close the lid or cover with foil until butter mixture is melted.