Open the tuna and drain well. Add the tuna to a mixing bowl and break it up using a fork. Trim the ends and finely chop the green onions, then add them to the tuna, mixing to combine.
In a small mixing bowl, whisk together the mayonnaise, oil, sriracha, and celery salt to create a blended mixture. Using a spatula, add the mayonnaise mixture to the tuna and mix until the tuna is fully coated and well combined.
Toast the bread until golden. Top each slice with a quarter of the tuna mixture and place on a baking sheet. Top each sandwich with a slice of provolone cheese and place on a rack in the top third of the oven. Turn on the broiler to high, and broil for 2 to 3 minutes, until the cheese is melted.
Remove from the oven, transfer to a serving plate, and sprinkle with a pinch of celery salt to garnish. Serve immediately.
Once assembled, the tuna melts are best enjoyed the day they are made. The tunamixture will keep for up to 3 days in the fridge when stored in an airtight container