Pat the T-Bone steaks dry with paper towels. Rub both sides with olive oil, then season generously with Montreal steak seasoning, salt, and pepper. Allow to sit at room temperature for about 30 minutes.

Wash and peel the russet potatoes. Using a mandoline slicer, slice the potatoes into thin lattice shapes. Soak the slices in cold water for at least 30 minutes to remove excess starch. Drain and pat dry.

In a large heavy-bottomed pot, heat vegetable oil over medium-high heat until it reaches 350°F (175°C). Fry potato slices in batches until golden brown and crispy, about 3-4 minutes. Remove with a slotted spoon and drain on paper towels. Season with salt to taste.

Preheat a grill or cast-iron skillet over high heat. Cook the seasoned T-Bone steaks for about 4-5 minutes on each side for medium-rare, adjusting time for desired doneness. Let rest for about 5 minutes before slicing.

In a saucepan, melt butter over medium heat. Add sliced onion and cook, stirring frequently, until caramelized and golden brown, about 10-15 minutes. Stir in minced garlic and cook for an additional minute.

Pour in beef stock and Worcestershire sauce, then add thyme leaves. Simmer for about 5 minutes to reduce slightly. Season with salt and pepper to taste.

Slice the rested T-Bone steak and arrange on a plate. Serve with crispy lattice fries on the side and drizzle the velvety onion brown sauce over the steak. Enjoy!