3 tablespoons spicy brown mustard
1/4 cup honey
1 garlic clove
1/2 cup mayonnaise
1 beef tenderloin
5 large mushrooms, sliced
1 large vidalia or sweet onion, sliced 1/2-inch thick
1 cup baby spinach
1/4 cup crumbled blue cheese
1 ciabatta loaf, split lengthwise
3 tablespoons extra virgin olive oil
In a food processor, or blender, combine mustard, honey, garlic, and mayonnaise. Process or blend until smooth. Season with salt and pepper to taste, and set aside.
In a large cast-iron skillet, heat 1 tablespoon olive oil over medium-high heat. Add onions, season with salt and pepper, and cook, stirring occasionally until golden and translucent, about 20 minutes. Remove from pan and in same pan, heat another tablespoon of oil. Add mushrooms and cook, stirring often until soft, about 5 minutes. Set aside.
While you are cooking the onions, coat the tenderloin with 1 tablespoon olive oil and season well with salt and pepper. On a grill heated to medium-high, cook beef, turning occasionally to create a crust. Continue to grill until medium-rare, about 20 minutes. Transfer to a plate and allow to rest 10 minutes covered with foil. Before cutting into 1/2-inch slices.
Brush both sides of ciabatta with olive oil and grill, cut-side-down, until lightly charred, about 5 minutes.
To assemble: Spread aioli on both sides of bread. Top the bottom half with onions, then beef, then mushrooms, then blue cheese, then spinach. Finishing with top slice of bread. Cut into 4 sandwiches, and serve immediately.