Sunday Brunch: Mushroom Tart with Herb Salad

2 egg yolks

1 1/4 pound assorted mushrooms, cleaned and torn into large bite-size pieces

5 tablespoons extra-virgin olive oil

2 tablespoons unsalted butter

Kosher salt

Freshly ground black pepper

1 1/2 cups sliced young onions, plus

1/4 cup diagonally sliced young onion tops

1 teaspoon thyme leaves

1/2 cup whole milk ricotta, drained if wet

1/4 cup creme fraiche

1/4 pound Gruyere, thinly sliced or grated

1/2 cup flat-leaf parsley leaves

1/4 cup tarragon leaves

1/4 cup chervil sprigs or marjoram leaves

1/4 cup 1/2-inch snipped chives

1/2 lemon