In an 8-quart stock pot, melt butter and olive oil over medium heat. Add onions and continually stir until tender and translucent. Do not brown the onions.
Add water, Member's Mark™ Beef Base, sherry, and Member's Mark™ Thyme Leaves. Season with Member's Mark™ Black Peppercorn Grinder and Member's Mark™ Himalayan Pink Salt Grinder. Simmer for 30 minutes. Meanwhile, preheat the oven's broiler.
Ladle soup into oven-safe serving bowls. Place one slice of bread on top of each bowl. Layer each slice of bread with a slice of provolone, ½ slice diced Swiss, and 1 tablespoon Parmesan cheese.
Place bowls on a cookie sheet and broil in the preheated oven until cheese bubbles and browns slightly, 2 to 3 minutes.
Carefully remove from the oven, and enjoy.