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 gruyre, cover with plastic wrap, and chill overnight.
Preheat oven to 35
Remove plastic wrap from eggs and bake until set and top is golden brown, 40 to 45 minutes.