In a mini food processor, add the mustard, honey, garlic and mayonnaise processing until smooth. Season with salt and pepper to taste. Let sit for 30 minutes.
Coat the tenderloin in 1 tablespoon olive oil and cover liberally with salt and pepper. On a prepared grill, cook the beef over med-high heat, turning occasionally to create a crust. Continue to grill until medium rare, approximately 20 minutes. (use a grill thermometer to determine temperature, I remove around 125 degrees.) Transfer to a plate and allow to rest for 10 minutes covered with foil.
Drizzle the onions and mushrooms with olive oil and season with salt and pepper. Grill the onions and mushrooms until soft, approximately 5-7 minutes. Transfer to a plate.
Brush both cut sides of the ciabatta with olive oil and grill, cut side down, until lightly charred, approximately 5 minutes.
To assemble: Slice the beef into 1/2 inch slices, set aside. Brush both cut sides of the bread with aioli, top the bottom half with grilled onions and then layer the beef on the onions. Top the beef with the mushrooms, blue cheese, spinach and then the top of the ciabatta. Cut into four sandwiches, serve immediately.