Crust
2 cups (250 grams) all-purpose flour
1/4 teaspoon table salt
1/2 cup (1 stick, 4 ounces or 113 grams) chilled butter, in cubes
3 tablespoons cold water

Filling
1 1/2 pounds yellow onions (about 4 medium), halved and thinly sliced
1 tablespoons butter
1 tablespoon olive oil
Scant 1/2 teaspoon table salt
Pinch of sugar
1 cup low-sodium beef, veal or mushroom stock/broth
2 teaspoons cognac, brandy or vermouth (optional)
Freshly ground black pepper
1/2 cup (about 2 ounces or 60 grams) grated Gruyere, Comte or Swiss cheese
1 large egg
1/2 cup heavy cream (half-and-half and milk work too, but cream tastes best)