1 pound penne (or any bite-size pasta shape such as ziti)
2 tablespoons olive oil, divided
4 tablespoons unsalted butter, plus more for buttering the dish
1 large onion, diced
1/4 cup all-purpose flour
2 cups milk
2 cups heavy cream
3 cups (12 ounces) grated Wisconsin Gruyère Cheese
1/2 cup (about 2 ounces) grated Wisconsin Parmesan Cheese
2 5-ounce bags baby spinach (about 10 loosely packed cups)
1 tablespoon hot sauce (such as Texas Pete® or Tabasco®)
2 tablespoons fresh marjoram leaves (or 2 teaspoons dried)
2 teaspoons sea salt
1/2 teaspoon freshly ground black pepper