Bring the steak to room temperature and season both sides liberally with black pepper and salt. Cover with plastic wrap and set aside.
Meanwhile, heat the 2 tbsp (30ml) of olive oil in the skillet over medium heat. Add the thyme sprigs and the onions. Stir and slowly cook until the onions are golden brown and caramelized in flavor for approx 15 to 20 min. Season with salt and pepper. Turn off heat, discard the thyme sprigs. and let sit while the steak cooks.
Heat the grill to medium high heat. Once up to heat, add the steak. Cook for 8 min on each side for medium rare, longer if well done. Tent the steak with foil and let rest for 10 min.
Grill the bottom halves of the baguette for 1-2 min or until well grilled. Set aside the top halves for another use.
Slice the steak thinly against the grain. Assemble the sandwiches by dividing the aioli evenly, spreading across the baguette. Add about a quarter of the sliced steak over each baguette on top of the aioli.
Top the steak with the caramelized onions, the Lakeside Heirloom Tomato slices, and the baby arugula.
Season tops of sandwiches well with fresh black pepper and kosher salt.