In a sauté pan over medium heat, add the canola oil and cook the bacon until golden brown and crispy, 8 to 10 minutes. Transfer the bacon to a plate lined with paper towels. In the same pan with the rendered bacon fat, add the sliced onions and caramelize to a deep mahogany color, 20 to 25 minutes.
Assemble the panino: Build the sandwich from the bottom up. Brush a slice of the sourdough bread with half of the butter. Flip the slice over and spread the mayonnaise evenly on top. Add the remaining ingredients in the order listed, top off with the second slice of sourdough and brush it with the remaining butter. Using a panini press, toast the panino until the cheese is melted, the turkey is fully warmed through and the bread is golden brown, 10 to 12 minutes. Serve immediately.