Pre heat oven to 400°
Slice mushrooms.
Puree ricotta in processor until smooth, add 1 egg yolk and 2 teaspoons oil and blend. Transfer ricotta to a bowl and fold in sour cream.
Heat remaining 2 tablespoons of oil in a skillet over medium high heat, add mushrooms and sauté for 6-7 minutes. Mix in thyme and season with salt and pepper. Add butter, mix in green onions and sauté until mushrooms and onions are tender, about 4 minutes.
Roll out puff pastry to 13x9 inches. Using a sharp knife, cut 1/4" score line all around the edge of the pastry. Brush the 1/4" border with egg yolk/water wash. Transfer pastry to baking sheet.
Spread ricotta mixture over dough inside the border. Top with half of the mushrooms, half of the Gruyère, then repeat with the balance, ending with Gruyère.
Bake until crust is golden and cheese melts; about 25 minutes.