Grilled Tuna and Vegetables with Fresh Herb Mayonnaise

Name: Grilled Tuna and Vegetables with Fresh Herb Mayonnaise
Ingredients: 1 1/2 lb. (750 g) fingerling potatoes, halved lengthwise; 1/4 cup (2 fl. oz./60 ml) extra-virgin olive oil; 3 Tbs. fresh lemon juice; 2 Tbs. minced fresh marjoram; 1 tsp. grated lemon zest; 1 shallot, minced; 1/2 cup (4 oz./125 ml) mayonnaise; 4 albacore tuna steaks, each 5 to 6 oz. (155 to 185 g); 1 1/4 lb. (625 g) assorted summer squash, cut diagonally into 1/3-inch (9-mm) slices; Kosher salt and freshly ground pepper