Mustard Greens Tart with Caramelized Onions and Gruyere

Name: Mustard Greens Tart with Caramelized Onions and Gruyere
Ingredients: 1 sheet (about 8 oz) frozen all-butter puff pastry, slightly thawed; 2 egg yolks; 4 tablespoons plus 1 teaspoon olive oil; 3/4 pound yellow onion, thinly sliced (about 3 cups); 2 teaspoons thyme, finely chopped; 1 bunch mustard greens, stems and center ribs discarded, leaves cut into wide ribbons; 2 large garlic cloves, minced; 1/4 cup whole milk ricotta; 2 tablespoons whole milk greek yogurt; 1/2 cup gruyere cheese, shredded; kosher salt and pepper, to taste; 1/2 lemon, for juicing