While the potatoes are in the oven, place a large, nonstick skillet over medium heat with 4 tablespoons of butter. Once melted, add onions, minced garlic, bay leaf, ground thyme and some salt and pepper. Cook onions for about 10-12 minutes, until caramelized. Add sherry and let the alcohol cook out, about 3-4 minutes. Remove the bay leaf and set the mixture aside to top the burgers.
In a medium bowl, combine the ground sirloin, Worcestershire, parsley, salt and pepper. Divide the mixture into 4 equal portions and form 4 patties, each about an inch thick. Drizzle the patties with EVOO and place them on the preheated grill pan. Cook about 5 minutes per side, until pink in the center.
While the burgers are cooking, place a small sauce pan over medium heat, add the remaining 2 tablespoons of butter. Once the butter is melted, add the shallots and cook for 1-2 minutes. Add the flour to make a roux. Cook the flour-butter mixture for about 3-4 minutes, while stirring constantly, until lightly golden in color. Whisk in the beef stock, a little at a time to avoid lumps. Once all of the stock is added, stir in the Dijon mustard and season with salt and pepper. Turn heat to low and set aside.
For the French baguette, you want large round slices: Hold your serrated knife on a 45-degree angle and slice, keeping your knife on that angle; just like slicing a flank steak or London broil. Drizzle bread slices with EVOO and season with salt and pepper. Place under the broiler for about 30 seconds. Flip to the other side and top four of the slices with the shredded gruyere cheese. Give them another 20 seconds under the broiler to give them a good melt. Keep a close eye on it, because if you're like me, you've burned one too many loaves of bread!
Last, but not least, add the sack of mixed greens to a salad bowl and drizzle with the red wine vinegar and a few tablespoons of EVOO. Season with salt and pepper and toss to coat.
It's burger assembly time! Place a burger on a piece of non-cheesy bread. Top with the caramelized onion mixture and a slice of the cheesy bread. Place a serving of potatoes on the plate and spoon some of the gravy on top. Place a portion of the mixed greens salad on the plate and enjoy!