In a medium bowl, combine the horseradish, lemon juice, vinegar, 1/4 teaspoon salt and 1/8 teaspoon black pepper; whisk in the extra virgin olive oil; set aside.
In a small bowl, mix together the barbeque sauce and honey mustard; set aside.
Place bacon into a medium skillet over medium heat. Cook until crispy, approximately 3 - 4 minutes per side. Drain; set aside.
Place skillet with bacon drippings back over medium heat. Add in the onion and 1/4 teaspoon salt and cook until they just begin to soften. Add in the mushrooms, continue to cook until mushrooms are softened. Remove from skillet; set aside.
Meanwhile, season ribeye with remaining 1/4 teaspoon salt and remaining 1/8 teaspoon black pepper. Place a grill pan over medium high heat. Add ribeye and cook to desired doneness. Remove from pan, cover and let rest for a few minutes.
Spread the butter onto the cut sides of the kaiser roll, sprinkle with the garlic powder. Place on the grill pan (still over medium heat) and grill until toasty.
Add the baby greens to the reserved horseradish vinaigrette, toss to coat. Thinly slice the ribeye.
To build your sandwich, place the bottom of the roll cut side up onto work surface, spread with half of the bbq/mustard mixture, top with the ribeye slices, blue brie, bacon, onion and mushrooms, and salad. Spread the cut side of the roll top with remaining bbq/mustard mixture, place onto sandwich and enjoy!!