In a large skillet, heat 1 tablespoon butter and olive oil over medium heat. Add onion, thyme, rosemary, bay leaf, brown sugar and pepper. Cover; cook over medium-low heat for 15-20 minutes, stirring occasionally, until onions are soft and lightly browned.
Meanwhile, in a small bowl, combine Parmesan cheese and remaining 4 tablespoons softened butter; set aside.
Remove the lid from the skillet; increase heat to medium. Add water and aus jus mix; simmer until reduced, 2-3 minutes. Remove from heat; discard bay leaf.
Preheat a large nonstick skillet or griddle over medium heat.
Top each slice of bread with two slices Swiss cheese and 1/4 of the onion mixture. Sprinkle each with 1/4 cup Gruyere; top with another slice of bread. Spread both sides of each sandwich with Parmesan butter.
Place sandwiches on the preheated griddle. Cook until golden brown and cheese is melted, 3-4 minutes per side. Remove to a cutting board; cut each sandwich in half. Serve immediately.