Preheat a charcoal or gas grill for indirect cooking over medium-high heat (375°). Spray bottom and sides of a 15x10-inch jelly roll pan with nonstick spray; set aside.

Melt butter in large skillet over medium heat. Add leeks, red onion, and ½ teaspoon salt. Cook 2 to 3 minutes or until softened, stirring occasionally. Add spinach and garlic. Cook 1 to 2 minutes or just until spinach begins to wilt. Remove from heat; cool slightly.

Whisk together egg, ricotta, and feta cheeses, lemon zest, pepper, and remaining ½ teaspoon salt in medium bowl; set aside.

Unfold puff pastry sheet onto lightly floured work surface; roll to 17x12-inch rectangle. Fold pastry in half and place in prepared pan. Unfold pastry and gently ease onto bottom and sides of prepared pan. Generously prick pastry with a fork.

Stir together ricotta and spinach mixtures; spread in pastry-lined pan to within ½-inch of bottom edges. Fold edges of pastry up and over filling to make a border.

Place pan over indirect heat. Close grill lid; grill 35 to 45 minutes or until pastry is golden brown. Let stand 10 minutes before serving.