Preheat oven to 400 degrees F.
Melt the butter in a sauce pot over medium heat, add the flour to form a roux. Slowly add the milk whisking constantly to avoid lumps. Stir in the mustard, herbs and nutmeg. Allow the sauce to thicken stiring frequently for 2 minutes. Season with salt and pepper if desired. Turn off heat and stir in gruyere.
In a pan, heat oil from sundried tomato jar. Sautée mushrooms for 3 minutes until browned. Add spinach and sundried tomatoes. Cook an additional 2 minutes.
When you are ready to serve, spread dijon mustard on the inside of 1 slice of bread. Add the mixed veggie filling, top with a scoop of mornay and close sandwich with other slice of bread. Transfer to baking tray and bake for 5 minutes until bread begins to turn golden and crisp. Remove from the oven, top with a bit more sauce and shredded gruyere. Return to the oven for an additional 5 minutes or until the cheese is nicely melted.
While the sandwiches are baking, fry and eggs to place on top. Garnish with avocado, greens and herbs.