There's not much cheese in this gratin — the better to let the perfectly ripe summer peppers and tomatoes, cooked just enough to intensify their flavors, sing.
Cooks' Notes:
· Stainless steel, glass, and enameled cast iron are nonreactive; avoid pure aluminum and uncoated iron, which can impart an unpleasant taste and color to recipes with acidic ingredients.
·Gratin can be baked 6 hours ahead and kept, uncovered, at room temperature.