Bonkers Awesome Mushroom And Onion Mac And Cheese

Name: Bonkers Awesome Mushroom And Onion Mac And Cheese
Ingredients: 4 tablespoons olive oil and 2 tablespoons unsalted butter, divided, for cooking the onions and mushrooms; 1 large yellow onion, sliced in semi-circles; half a pound of button or cremini mushrooms, cleaned and quartered; salt and pepper to taste; 1 teaspoon chopped fresh thyme, divided; 5 tablespoons unsalted butter, divided, plus a bit more for greasing the pan; 1/4 cup all-purpose flour; 2 1/2 cups whole milk; 1 large egg; 1 scant teaspoon salt; 1/2 teaspoon freshly ground black pepper; 4 ounces goat cheese; 1/2 cup shredded Gruyère; 1 1/2 cups shredded sharp cheddar cheese; 1 pound pasta shells, boiled until just undercooked; 1/3 cup panko crumbs