Wash chard and trim discolored ends of stems. Cut out central stalk from leaves with a V-shaped incision that follows the tapering shape of the stalk. Thinly slice stalks and set aside. Chop leaves.
In a large frying pan over medium-high heat, melt 2 tbsp. butter. Add chard stalks and 1/2 tsp. salt. Cook, stirring occasionally, until tender. Add chard leaves and garlic. Continue to cook, stirring occasionally, until leaves are tender. Stir in white pepper. Set aside and let cool slightly.
Butter a 9- by 12-in. baking dish. Spread cooked chard on bottom and sprinkle with ricotta salata. In a large bowl, whisk together eggs, cream, and remaining 1/2 tsp. salt. Pour over chard and ricotta salata. Sprinkle with gruyère, cover with plastic wrap, and chill overnight.
Preheat oven to 350°. Remove plastic wrap from eggs and bake until set and top is golden brown, 40 to 45 minutes. Serve hot, cut into squares.
Note: Nutritional analysis is per serving.