Heat 1 tbsp. canola oil in a medium skillet. Add sliced yellow onions and sauté until soft and translucent but not caramelized. Transfer to separate bowl and set aside.
Add remaining oil to skillet and heat until shimmering. Add thinly sliced chicken and cook until golden, 1-2 minutes.
Top chicken with ¼ cup buffalo sauce and stir to coat.
To assemble: Open hoagie rolls to lay flat, keeping the hinge intact. Add 2 slices of provolone cheese to each roll; top with buffalo chicken, sautéed onions, a drizzle of blue cheese, scallions, and additional buffalo sauce, if desired.