Similar to aioli, but without the harshness of garlic, this lavish sauce, flavored with lemon and fresh herbs, is a handy gluten-free staple to keep in your refrigerator. It is an elegant accent for grilled albacore and squash.
In a medium pot of boiling salted water, blanch the potatoes until almost tender, about 10 minutes. Drain.
Meanwhile, in a small bowl, mix the oil, lemon juice, 1 tablespoon plus 2 teaspoons of the marjoram, lemon zest, and shallot. In another small bowl, combine 1½ tablespoons of the mixture and the mayonnaise.
Place the tuna and squash on a baking sheet. Brush with the olive oil mixture and sprinkle with salt and pepper. In a medium bowl, mix the potatoes and remaining olive oil mixture. Sprinkle with salt and pepper.
Prepare a grill for direct-heat cooking over high heat. Add the tuna, squash, and potatoes. Cover and grill just until cooked through, about 5 minutes per side. Transfer the tuna and vegetables to 4 warmed plates. Sprinkle with the remaining 1 teaspoon marjoram. Serve right away with the mayonnaise-herb mixture.